Image forming apparatus

An image forming apparatus comprising:

• an image bearing member configured to bear a toner image;
• an intermediate transfer belt configured to bear the toner image transferred from the image bearing member;
• a separation mechanism configured to separate the intermediate transfer belt from the image bearing member;
• a driving unit configured to drive the separation mechanism;
• a cam configured to be driven by the driving unit to operate the separation mechanism;
• a phase detection portion configured to detect a phase of the cam;
• an optical sensor configured to irradiate a control toner image formed on the intermediate transfer belt with light and detect reflection light of the light, the optical sensor being configured such that a distance between the optical sensor and the intermediate transfer belt changes in accordance with an operation of the separation mechanism;
• and
• a controller configured to execute an abnormality diagnosis mode in a case where change in the phase of the cam is not detected by the phase detection portion until an elapse of a predetermined time or more since a start of output of a driving signal for driving the separation mechanism,
• wherein, 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 the 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.