rotor shaft 3 the v belt 40 is disposed around both of the pulleys 20 and 30 to link them together in order to rotate the driven pulley 30 with the driving pulley 20 the tension pulley 50 is carried by a lever 51 which is pivoted by a pin 52 fixed to the engine block being biased by a spring 53 to contact with and push the v belt 40 so as to give a suitable tension to the v belt 40 the driving pulley 20 includes a pulley disk 200 fixed to the driving shaft 1 of the engine to the periphery of which a generally cylindrical member or a first pulley plate 210 is secured at its one end the first pulley plate 210 has a first slope portion 211 formed on the other end thereof a slidable helical gear 220 is carried by the driving shaft 1 having an inner straight spline 221 engaging with an outer straight spline 5 formed on the driving shaft 1 the helical gear 220 is slidable relative to the driving shaft 1 but is rotated integrally with the shaft by means of the straight splines to one end portion of the helical gear 220 is fixed a generally dish shaped member or a second pulley plate 230 which has on its periphery a second slope portion 231 facing to the first slope portion 211 of the first pulley plate 210 the slope portions 211 and 231 of the first and the second pulleys define therebetween an.