To provide a power transmission mechanism with easy application in response to purpose interposed between an input shaft side and an output shaft side, suitable when providing constant rotation of the output shaft side in the same direction by normal/reverse rotation of the input shaft side, and capable of improving operation efficiency and operability.

A gear assembly which is a one-way clutch is composed having a first gear 20 forming external teeth 23 on a peripheral wall 21b, a second gear forming external teeth 27 on a peripheral wall, or the like, inoperable connection is carried out when the first gear 20 is rotated in one direction with respect to the second gear 25, and operable connection is carried out when the first gear 2 is operated in another direction with respect to the second gear 25. In the power transmission mechanism 1, the gear assembly 2 is used in a pair, it has a rotatably supported idle gear 3, outer teeth 23 of each first gear 20 of both gear assemblies 2A and 2B are meshed with each other, and outer teeth 27 of each second gear 25 of both gear assemblies 2A and 2B are respectively meshed with the idle gear 3.