An electric power steering device (1) is provided which transmits output rotation of an electric motor for generating a steering assist force (16) to a steering mechanism (64) via a driving gear (22) and a driven gear (21). A housing (8) has a support hole (28) for supporting one end portion (23) of a shaft (20) co-rotatable with the driving gear (22) via a bearing (26) in such a manner as to permit the one end portion (23) to be offset toward the driven gear (21) radially of the driving gear (22). A biasing member (33) which biases the driving gear (22) toward the driven gear (21) includes an arcuate main portion (47) fitted around an outer peripheral surface (58) of an outer ring (36) of the bearing (26) and opposed to an inner peripheral surface (42) of the support hole (28). A pair of movement restriction portions (50, 51) which restrict circumferential movement of the main portion (47) are respectively provided at a pair of arc ends (48, 49) of the main portion (47). A resilient tongue (52) which biases the driving gear (22) toward the driven gear (21) via the bearing (26) is provided only on one of the movement restriction portions (50).