In a power transmission mechanism 20, a line connecting a center shaft 21 of a reference gear member 40 and a first shaft 44 of a first planetary gear member 42 and a line connecting the center shaft 21 of the reference gear member 40 and a second shaft 47 of a second planetary gear member 45 form a specific angle α. Rotation in one direction is prevented when a first carrier member 22 and a second carrier member 32 are relatively rotated in a direction to decrease the specific angle α. Further, a rotation prevention release unit is provided to release a prevention state of the rotation when the first carrier member 22 and the second carrier member 32 are relatively rotated in a direction to increase the specific angle α.