PURPOSE: To detect the position of a sensor supporting arm without providing a specialized detecting means by selecting a combination of sensors that are not used for detecting any original size and judging the retreat of the sensor supporting arm when the combination detects a part to be detected.

CONSTITUTION: The title device is provided with the part to be detected, which is placed at the prescribed location of a device main body frame, and a judging means which selects grouped sensors that are not used for detecting any original size among plural sensors 46-49 and judges that the sensor supporting arm retreats when the grouped sensors detect the part to be detected. Namely, when the sensor supporting arm 40 retreats, the grouped sensors that are not used for detecting original size detect the part to be detected, and the judging means judges that the sensor supporting arm 40 retreats. As a result, the judging means can detect that the sensor supporting arm retreats without providing the original discriminating sensor device with another detecting means such as a sensor.