A hybrid drive system has an engine (11); a motor-generator (16); a transmission (12) for converting drive force inputted from at least one of the engine (11) and the motor-generator (16) and transmitting the converted drive force to a drive wheel (15); a first rotating shaft (41), a second rotating shaft (42) and a third rotating shaft (43) disposed mutually in parallel. The drive system includes a first gear wheel (21) disposed coaxially with the first rotating shaft (41); a first clutch (31) for engaging/disengaging the first gear wheel (21) with/from the first rotating shaft (41); a second gear wheel (22) disposed coaxially with the second rotating shaft (42); a second clutch (32) for engaging/disengaging the second gear wheel (22) with/from the second rotating shaft (42); a third gear wheel (23), engaging with the first gear wheel (21) or the second gear wheel (22) and disposed coaxially with the second rotating shaft (42) or the first rotating shaft (41); and a third clutch (33) for engaging/disengaging the third gear wheel (23) with/from the first rotating shaft (41) or the second rotating shaft (42). The motor-generator (16) is connected to the first rotating shaft (41). The engine (11) is connected to the third rotating shaft (43). The transmission (12) includes an input disc (12a) connected to the third rotating shaft; an output disc (12b) disposed coaxially with the input disc; and a power roller (12c) sandwiched in a tiltable fashion between the input disc and the output disc.