A power transmission device includes a first one-way clutch (1) that transmits a rotational power of a pulley (3) to a rotating shaft (6), and a second one-way clutch (2) that transmits a rotational power of a rotor (11) of a motor (10) to the rotating shaft (6). The first one-way clutch (1) is of inner cam type in which an engagement cam-surface is formed in an outer periphery of a first inner ring (7), while the second one-way clutch (2) is of outer cam type in which an engagement cam-surface is formed in an inner periphery of a second outer ring (14). This arrangement improves engagement performance of the first one-way clutch (1) as well as reducing heat generation of the second one-way clutch (2), so that the lifetime of grease is prolonged.
