A power transmitting device 1 having: first and second main input shafts 11, 12 connected to engine 2 via clutches C1 and C2; idler shaft 17 parallel to the first main input shaft 11; a sub input shaft 13 parallel to the idler shaft 14; gears 17a and 17b of an output shaft 17 allowing gears 18a and 18b disposed on the first input shaft 11 and selectively connected to the output shaft 17, and gears 19a and 19b able to connect the sub input shaft 13 and the output shaft 17, to engage with gears 17a and 17b in common; and a differential rotation mechanism 9, allowing a sun gear 9s on the first input shaft 11 and an electric motor 3, a carrier 9c connected to gear 18a, and a ring gear 9r connected to a synchronizer SL to rotate differentially with respect to one another.