mono fiber or a glass rod whose end face is a curved surface is used as shown in fig9 and 10 if the inclination angle of the light which passes through the reflection member is large there occurs a leakage of the light from the reflection member that is as shown in fig1 the rays which pass through the core b penetrate through the cladding if the inclination angle is larger than the value u03b1 1 derived from the following equation where n is the refractive index of the core b and n 39 is the refractive index of the cladding further it is to be noted that the cladding does not have to be used and in such a case n 39 1 0 applies to the above equation therefore the inclination angle of the light passing through the core of the cylindrical reflection member is smaller than the value u03b1 1 derived from the above equation when the rays of the inclination angle u03b1 1 come out to the air from the boundary a between the core and the cladding it is necessary that for rays to come out of the core without the total reflection at the surface s the radius curvature r of the surface s be within the following ranges equ1 where dc is a diameter of the core that is to say that when the radius curvature r of the surface s does not satisfy the above relation there is a loss in the amount of the light because the light coming out from the cylindrical reflection member is totally reflected at the.