An optical sensor irradiates a control toner image formed on an intermediate transfer belt with light and detects reflection light of the light. A controller executes an abnormality diagnosis mode in a case where change in the phase of a cam is not detected by a phase detection portion until an elapse of a predetermined time or more since a start of output of a driving signal for driving a separation mechanism. In execution of the abnormality diagnosis mode, the controller outputs the driving signal for driving the separation mechanism, and is capable of outputting information about an abnormality of the phase detection portion and information about an abnormality of the separation mechanism and/or a driving unit on a basis of a detection result of the optical sensor that is obtained in a case where the light is radiated from the optical sensor while the driving signal is output.