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).