When a trailing edge detection sensor is affected by toner, dust, etc. adhered on an optical path thereof or when the sensor fails, malfunction of the apparatus is prevented from happening by measuring the size of a record material only with a unit for detecting a leading edge. The image forming apparatus has a unit for leading edge detection adapted to detect the leading edge of the record material and a unit for trailing edge detection adapted to detect the trailing edge of the record material, and is configured to, when the unit for trailing edge detection falls in detection capability or fails, form an image and measure the size of the record material only with the unit for leading edge detection.