An image forming apparatus includes an apparatus main body, a sheet storage cassette, a sheet feed unit, a first detection sensor having a first detector for detecting the presence/absence of sheets stacked on a sheet stacking plate, a second detection sensor having a second detector for detecting the position of the top face of the sheets stacked on the sheet stacking plate, a cassette detection sensor, a driving device, and a controller. The sheet feed unit includes a first actuator for switching detection states of the first detection sensor and a second actuator for switching detection states of the second detection sensor. The controller determines the mounting state of the sheet feed unit based on the detection state of the first and second detection sensors.