PURPOSE: To prevent the separation failure of a form, by providing a light emitting element and a photodetector in the lower stream of a separating corotron, and detecting the separation position of the form on a basis of the intensity of the light, which is reflected by the surface of the form, to control a separation parameter.

CONSTITUTION: A light emitting diode 17 and a reflective photo sensor 18 which detects the light reflected by a form 21 are provided in the lower stream of a separating corotron 4. Since the intensity of the reflected light incident to the sensor 18 is changed in accordance with the sum between the length from the light emitting diode 17 to the form 21 and the length from the form 21 to the sensor 18, the separation position of the form is detected on a basis of the intensity of the reflected light. Consequently, form separation parameters such as the corotron current are controlled on a basis of the output of the sensor 18 to correct the form separation position to the normal position, and thus, the separation failure is prevented.