A novel sensor unit actuating mechanism that can cause a sensor unit to leave or enter a frame smoothly even when the thickness of the frame is reduced and that has a mechanism for coping with a prank by a child or the like, such as a push-back of the sensor unit. The sensor unit actuating mechanism includes a frame, a sensor unit including an optical sensor, a spring member configured to expand or contract in a Y direction, actuators configured to contract against the resilience of the spring member when energized, and a guide member for moving the sensor unit linearly in an X direction. When the actuator is energized, the sensor unit, is linearly moved from inside the frame to the measurement position; when the actuator is energized, the sensor unit is linearly moved from the measurement position to inside the frame and stored therein.