A mechanism for causing a movable body to self-propel forward has a runner (5) having a striker (51) on its lower side, a catcher (6) facing the striker (51) and engaged at a reference position R with the movable body (1) side, and biasing means (7) for storing biasing force when the catcher (6) is positioned at a reference position R. The catcher (6) at the reference position R arrests the striker (51) of the runner (5) engaged with and stopped by the rail (2) side at the self-propel startposition SA of the movable body (1). Upon the arrest of the striker (51), the engagement between the catcher (6) and the movable body (1) is released.At the same time, the catcher (6) is set at the reference position R again by a return travel of the movable body (1) from the stop position SP, the striker (51) of the runner (5) is released, and after that the engagement between the runner (5) andthe rail (2) is released.