A lock mechanism for a stage apparatus, the stage apparatus having a stationary support board and a movable stage relatively movable in a plane parallel to the stationary support board, includes a pair of engaging members projecting from the movable stage; a rotational lock device including a pair of arm members rotatably mounted and relatively rotatable between an engaged position at which facing free ends of the arm members clasp corresponding engaging members, and a disengaged position at which the facing free ends of the arm members move away from the corresponding engaging members; a lock driving device which rotates the arm members to the engaged position upon the movable stage entering a non-operational state; and an unlock driving device which rotates the arm members to the disengaged position so that the movable stage enters an operational state.