A hybrid electric drive system for a vehicle comprises a combustion engine (1), a transmission (2), a first clutch (3) disposed between the engine (1) and the transmission (2), a rotating electric machine (4) which serves as a motor as well as a generator, a second clutch (30) disposed between the rotating electric machine (4) and the transmission (2), and a storage device (9). When the second clutch (30) is commanded to connect the rotating electric machine (4) to the transmission (2), a control unit (10) controls the rotating electric machine (4) so as to decrease the difference between the rotation speeds of the rotating electric machine (4) and the transmission (2) (S2) and cause the second clutch (30) to connect the rotating electric machine (4) to the transmission (2) only after the difference decreases to a allowable value (S3, S4).