Stacking device and image forming apparatus

An image forming apparatus comprising:

• a stacking plate on which a sheet is to be stacked;
• a lift up device including a lifter gear and a lifter member, wherein the lifter member is configured to move with a rotation of the lifter gear and the lifter member is configured to raise and lower the stacking plate with the sheet;
• a feeding member configured to feed the sheet stacked on the stacking plate;
• an image forming unit configured to form an image on the sheet fed by the feeding member;
• a regulation member configured to be movable in a first direction and a second direction opposite to the first direction and configured to regulate a position of the sheet by abutting on an edge of the sheet stacked on the stacking plate;
• a first member configured to be detected and configured to move in a direction parallel to a moving direction of the regulation member in conjunction with the regulation member;
• a second member, provided on the first member, and configured to be detected and configured to move with respect to the first member;
• single sensor having an output configured to change based on a position of the first member and on a position of the second member;
• a determination unit configured to determine a size of the sheet based on the output of the sensor;
• and
• a contacting portion configured to contact the second member,
• wherein the second member is configured to be in a first position facing the single sensor or a second position not facing the single sensor according to the movement of the regulation member, and
• wherein, when the second member is located at the second position, the second member moves to a retraction position retracting from the lifter member by contacting the contacting portion.