Sensor unit and image forming apparatus

An image forming apparatus for forming an image on a sheet, comprising:

• an image forming portion configured to form the image on the sheet;
• and
• a sensor unit configured to detect a jam of the sheet, said sensor unit comprising,
• a rotatable first movable member,
• a sensor configured to detect a position of said first movable member, and
• a second movable member having a guiding function of guiding movement of the sheet,
• wherein said first movable member includes a member-to-be-detected configured to be detected by said sensor and an urging member configured to urge said member-to-be-detected toward a position in which a phase of said member-to-be-detected relative to said first movable member is a first phase,
• wherein when the sheet is normally fed along said second movable member, said first movable member is in a first position, said member-to-be-detected is in the first phase, said second movable member is in a third position, and a functional/non-functional state of said member-to-be-detected relative to said sensor is one of functional and non-functional states,
• wherein when the sheet is not fed and said first movable member is in a second position different in rotational direction from the first position, said member-to-be-detected is in the first phase, said second movable member is in the third position, and the functional/non-functional state of said member-to-be-detected relative to said sensor is the other of the functional and non-functional states, and
• wherein when said first movable member is in the first position and said second movable member is moved to a fourth position different from the third position by being pushed by a jammed sheet, by movement of said member-to-be-detected to a second phase different from the first phase through pushing thereof by said second movable member, the functional/non-functional state of said member-to-be-detected relative to said sensor is changed from the one state to the other state and an output of said sensor changes.