A sheet processing apparatus, comprising: a main body including an insertion slot; a sheet cassette configured to be inserted into the main body via the insertion slot and removed from the main body via the insertion slot, the sheet cassette including a pin member extending in a first direction orthogonal to an insertion direction of the sheet cassette, the pin member being connected to the sheet cassette so as to be moveable in a second direction orthogonal to the first direction and the insertion direction; and a housing in the main body including: a lever partially rotatable about a first axis proximate to a first end of the lever, the first axis being parallel to the first direction, the lever having a pin engagement portion at a tip portion at a second end of the lever opposite the first end; an elastic member configured to apply a force to the lever to rotate the lever toward the insertion direction; and a chassis having a guide groove formed therein, the guide groove extending along the insertion direction and controlling the position of the pin member in the second direction, the guide groove including an inclining portion that is inclined with respect to the insertion direction and is farther along the insertion direction than an intake portion of the guide groove for receiving the pin member into the guide groove, characterized in that the pin engagement portion is rotatable about a second axis parallel to the first direction and having an engagement groove into which the pin member can be inserted from a first pin engagement position and a second pin engagement position when the lever has rotated about the first axis, and the elastic member is connected to the lever at a position between the first axis and the pin engagement portion. a main body including an insertion slot; a sheet cassette configured to be inserted into the main body via the insertion slot and removed from the main body via the insertion slot, the sheet cassette including a pin member extending in a first direction orthogonal to an insertion direction of the sheet cassette, the pin member being connected to the sheet cassette so as to be moveable in a second direction orthogonal to the first direction and the insertion direction; and a housing in the main body including: a lever partially rotatable about a first axis proximate to a first end of the lever, the first axis being parallel to the first direction, the lever having a pin engagement portion at a tip portion at a second end of the lever opposite the first end; an elastic member configured to apply a force to the lever to rotate the lever toward the insertion direction; and a chassis having a guide groove formed therein, the guide groove extending along the insertion direction and controlling the position of the pin member in the second direction, the guide groove including an inclining portion that is inclined with respect to the insertion direction and is farther along the insertion direction than an intake portion of the guide groove for receiving the pin member into the guide groove, characterized in that a lever partially rotatable about a first axis proximate to a first end of the lever, the first axis being parallel to the first direction, the lever having a pin engagement portion at a tip portion at a second end of the lever opposite the first end; an elastic member configured to apply a force to the lever to rotate the lever toward the insertion direction; and a chassis having a guide groove formed therein, the guide groove extending along the insertion direction and controlling the position of the pin member in the second direction, the guide groove including an inclining portion that is inclined with respect to the insertion direction and is farther along the insertion direction than an intake portion of the guide groove for receiving the pin member into the guide groove, characterized in that the pin engagement portion is rotatable about a second axis parallel to the first direction and having an engagement groove into which the pin member can be inserted from a first pin engagement position and a second pin engagement position when the lever has rotated about the first axis, and the elastic member is connected to the lever at a position between the first axis and the pin engagement portion.