Thickness detection device, sheet processing apparatus, and image forming system

A thickness detection device to detect a thickness of an object, the thickness detection device comprising:

• a thickness-direction moving member configured to move in a direction of the thickness by the object, wherein
• the thickness detection device defines a cavity and an opening connected to the cavity,
• the cavity is configured to receive the object and the thickness-direction moving member is configured to move in response to the object being received in the cavity, and
• the opening opens in a plurality of directions;
• and
• a thickness detector configured to detect the thickness-direction moving member moved in response to the object being received in the cavity to detect the thickness of the object, wherein
• the thickness-direction moving member has a portion that faces the object being received in the cavity, the portion having a shape inclined relative to an insertion direction of the object,
• the thickness-direction moving member is configured to be biased in a thickness direction of the object, and
• the cavity is defined by a surface of the thickness detection device, and the surface defines a depression in the cavity, the depression being recessed in a biased direction of the thickness-direction moving member, and the depression being configured to receive at least a portion of the thickness-direction moving member in the depression in absence of the object in the cavity.