A sheet transport path switching mechanism includes a movable guide to be moved rotatably about a support shaft parallel to the width direction of a transport path so as to selectively take either a main posture where the movable guide directs a sheet to the downstream side of a main transport path relative to a branch point, and a branch posture where the movable guide directs a sheet to a branch transport path. An engagement hook is provided in an apparatus body and supports a central region in the longitudinal direction of the movable guide to prevent the movable guide from being deformed due to interference with a sheet fed toward the branch transport path. The mechanism prevents deformation of the movable guide while facilitating reduction in material and production costs and adequately meeting requirements of waste treatment.