PURPOSE: To a provide a power transmission device which is constituted to suppress the generation of gear impact noise and prevent the occurrence of wear.

CONSTITUTION: Constitution of a power transmission device is such that by boosting working oil in an oil pressure generating chamber by a driven gear and a ring gear according to a rotation difference between a casing and a ring gear, a power is transmitted between a casing and retainer 30. In the constitution of the power transmission device, a recessed part 30c to rotatably support the driven gear is formed in the outer peripheral part of the retainer 30 and a guide surface 55a with which a part of addenda 31a and 31a' are simultaneously brought into slide contact is formed on the opening edge 30d of the recessed part 30c.