A first friction clutch (54) is constructed as a negative type friction clutch and a second friction clutch (55) is constructed as a positive type friction clutch. Spring constant of a second spring (75) is made smaller than that of a first spring (65). Therefore, in the middle stage of switching the first and second friction clutches (54, 55), both of these first and second friction clutches (54, 55) enter in a connection state. Consequently, a driven member (56) is driven through either or both of the first and second clutches by a driving shaft, and the free rotation state does not occur.