To provide a power transmission mechanism capable of accurately managing rotation position even in a rotation drive force transmission release state, in a rotation drive member changing over rotation drive force from a drive source between a transmission state and the transmission release state.

This power transmission mechanism includes: a rotation drive member driving a driven member by rotation; a drive change over means capable of changing over between the transmission state in which the drive force of the drive source is transmitted to the rotation drive member to rotate the same, and the transmission release state in which drive force transmission to the rotation drive member is released; and a rotation regulating means allowing rotation of the rotation drive member when the drive change over means is in the transmission state, and engaging with the rotation drive member and regulating rotation when the drive change over means is in the transmission release state.