of two convex lenses 16 with respect to one laser light source 12. The focal position F is set on the surface T of the member S (CFRP member in the present exemplary embodiment) for which roughening treatment is performed. The configuration of the light collector 14 is not limited as well to that in the present exemplary embodiment. For example, a reflector may be used as the light collector 14, or alternatively, a lens and a reflector may be used in combination to form the light collector 14. 

[0021] The laser beam L irradiated from the laser light source 12 is refracted at the light collector 14 and is then concentrated on the focal position F, i.e., the surface T of the member S. 

[0022] Thewaveform ofthe laserbeamL irradiated from the laserlightsource 12willbe hereinafterexplained indetail. As shown in FIG. 2, the laserbeam L has a pulse-like waveform.One recess X is configured to be formed on the surface T of the member S in a single pulse cycle that the value of electric current increases from a baseline current B, and thereafter, reduces to the baseline current B again. Besides, the pulse cycles occur at intervals of predetermined time, and the laser beam L is irradiated in each pulse cycle. It should be noted that the value of the baseline current B may be zero, or alternatively, may be a predetermined value greater than zero. 

[0023] In the present exemplary embodiment, the laser beam L has a waveform composed of a former portion 100, an intermediate portion 102 and a latter portion 104 when the single pulse cycle thereof is focused. In the former portion 100, an increase rate of irradiation intensity (current intensity) per unit time is maximized on the outset of irradiation and is then graduallyreduced as irradiation intensity(currentintensity) approachesthe maximum. In the intermediate portion 102, the increase rate of irradiation intensity (current intensity) per unit time approaches zero. In the latter portion 104, a reduction rate of irradiation intensity (current intensity) per unit time is maximized after occurrence of the intermediate portion 102 and is then gradually reduced as irradiation intensity (current intensity) approaches the baseline current B. 

[0024] Through the entire specification of the present invention, the state appearing in the intermediate portion 102- the increase rate of irradiation intensity (current intensity) per unit time approaches zero-means that the increase rate of irradiation intensity (current intensity) per unit time becomes less than or equal to 10%, where the maximum increase rate thereof on the outset of irradiation is defined as 100%. 

[0025] Itshould be noted thatin the presentinvention,the intermediate portion 102 isnotindispensable as a constituent element of the waveform of the laser beam L; the waveform of the laser beam L may be composed of the former portion 100 and the latter portion 104. 

[0026] According to the present exemplary embodiment, the following configuration is performed: in the former phase of irradiating the laser beam L on the surface T of the member S, irradiation intensity (current intensity) is quickly increased,and then,the increase rate ofirradiation intensity (currentintensity)perunittimeisgraduallyreduced,whereas in the latter phase of irradiation, irradiation intensity (current intensity) is quickly reduced. Accordingly, as shown in FIG. 3, the recess X is formed on the surface T of the member S in the irradiation position of the laser beam L such that the middle part thereof is deeply recessed as a bottom Z. Moreover, the recess X is shaped to have a cross section gently slanting toward the bottom Z. 

[0027] Thus, with the configuration that the recess X is formed on the surface T of the member S while being shaped to have a cross section including a slope K gently slanting toward the bottom Z, even in applying adhesive Y with a certain high level of viscosity onto the surface T of the member S, the recess X enables the adhesive Y to flow to the bottom Z more easily than, for instance, a recess formed in use of a square-wave laser beam (see FIG. 4) so as to be shaped to have a cross section having an approximately rectangular shape (see FIG. 5). Because of this, adhesiveness on the surface T of the member S can be enhanced. 

[0028] It should be noted that as described above, when the laser beam L has a waveform composed of the former portion 100 and the latter portion 104, the recess X formed by the laser beam L has a cross section that the bottom Z is short in the width direction (alternatively, a cross section composed only of the gentle slope K almost without the bottom Z). Even in this case, adhesiveness on the surface T of the member S is still enhanced because the recess X has a cross section including the gentle slope K. However, compared to this, the configuration of the recess X having a cross section including the bottom Z (i.e., the configuration of the laser beam L having a waveform including the intermediate portion 102)ismore preferable in terms ofenhancementin adhesivenesson the surface T ofthe memberS. 

[0029] Besides, the increase rate of irradiation intensity (current intensity) per unit time is gradually reduced in the former phase of irradiation, whereby it is possible to avoid sudden rise of electric current, i.e., occurrence of current spike. Hence, even in use of a laser diode as the light source 12 of the laser beam L, the laser diode can be mitigated in damage. 

(Modification 1) 

[0030] In the exemplary embodiment described above, the laser irradiation device 10 is composed of a single pair of the laser light source 12 and the light collector 14. Alternatively, the laser irradiation device 10 may be composed of a plurality of pairs of the laser light source 12 and the light collector 14. 

[0031] As shown in FIG. 6, the laser irradiation device 10 according to modification 1 is composed of five pairs of the 

