An image forming apparatus includes discharging bins in which image-formed sheets are stacked, and uses a bin elevating motor to switch discharging into one discharging bin. The image forming apparatus is provided with a sensor for detecting the amount of stacked paper in one discharging bin to which discharging can be performed. When the operational discharging bin is changed, the discharging bins are moved by the bin elevating motor, whereby the sensor detects the amount of stacked paper in the discharging bin to which discharging can be performed after the moving. Subsequently, the sensor directs a start of image formation.