An assist mechanism aids movement of a retractable unit reciprocally moving between an open and a closed position and includes a pressing device, a stopper mechanism, an engagement member, a catch portion, and a release member. The pressing device presses the retractable unit toward the open position in which the unit is extended. The stopper mechanism controls and releases retraction of the retractable unit to the closed position in which the unit is retracted, and includes a stopper and a blocking member. The engagement member is provided in the retractable unit. The catch portion engages the engagement member. The release member releases a force pulling the retractable unit when the retractable unit is moved to the closed position.