A unit attaching structure configured such that a unit is detachable and attachable with respect to the unit attaching structure, the unit attaching structure comprising: an attachment part having a bottom wall and a side wall intersecting with the bottom wall, the attachment part being configured such that the unit is detachable and attachable with respect to the attachment part along a first direction perpendicular to the bottom wall; a guiding member (201, 202) extending in the first direction, the guiding member being configured to engage with the unit to guide the unit to the attachment part when the unit is to be attached to the attachment part; and a pressing part provided to the side wall and configured to press the unit attached to the attachment part, wherein the pressing part comprises: a pressing plate having a rectangular shape viewed from a direction perpendicular to the side wall, the pressing plate being movable between a first position and a second position, the pressing plate being configured to protrude from the side wall when located at the first position, the pressing plate being configured to protrude from the side wall by a less amount when located at the second position than when located at the first position, the pressing plate being configured to contact the unit when the unit is attached to the attachment part; and characterized by an urging member configured to urge the pressing plate from the second position toward the first position, wherein the pressing plate has a pressing surface directed to a second direction from the second position to the first position, a plurality of first conductive electrodes being arranged on the pressing surface in a third direction perpendicular to both the first direction and the second direction, wherein, when the unit starts contacting the pressing plate, the unit being configured to contact a contacting part, the contacting part being a part of a particular end part of the pressing plate on a side opposite to the bottom wall and shifted in the third direction with respect to a position of center of load of gravity by urging of the urging member to the pressing plate, wherein the unit is configured to move with contacting the pressing surface and to be attached to the attachment part after contacting the contacting part, and wherein a contacting range, in the third direction, of the unit and the contacting part is smaller than a contacting range, in the third direction, of the unit attached to the attachment part and the pressing plate. an attachment part having a bottom wall and a side wall intersecting with the bottom wall, the attachment part being configured such that the unit is detachable and attachable with respect to the attachment part along a first direction perpendicular to the bottom wall; a guiding member (201, 202) extending in the first direction, the guiding member being configured to engage with the unit to guide the unit to the attachment part when the unit is to be attached to the attachment part; and a pressing part provided to the side wall and configured to press the unit attached to the attachment part, wherein the pressing part comprises: a pressing plate having a rectangular shape viewed from a direction perpendicular to the side wall, the pressing plate being movable between a first position and a second position, the pressing plate being configured to protrude from the side wall when located at the first position, the pressing plate being configured to protrude from the side wall by a less amount when located at the second position than when located at the first position, the pressing plate being configured to contact the unit when the unit is attached to the attachment part; and characterized by an urging member configured to urge the pressing plate from the second position toward the first position, a pressing plate having a rectangular shape viewed from a direction perpendicular to the side wall, the pressing plate being movable between a first position and a second position, the pressing plate being configured to protrude from the side wall when located at the first position, the pressing plate being configured to protrude from the side wall by a less amount when located at the second position than when located at the first position, the pressing plate being configured to contact the unit when the unit is attached to the attachment part; and characterized by an urging member configured to urge the pressing plate from the second position toward the first position, wherein the pressing plate has a pressing surface directed to a second direction from the second position to the first position, a plurality of first conductive electrodes being arranged on the pressing surface in a third direction perpendicular to both the first direction and the second direction, wherein, when the unit starts contacting the pressing plate, the unit being configured to contact a contacting part, the contacting part being a part of a particular end part of the pressing plate on a side opposite to the bottom wall and shifted in the third direction with respect to a position of center of load of gravity by urging of the urging member to the pressing plate, wherein the unit is configured to move with contacting the pressing surface and to be attached to the attachment part after contacting the contacting part, and wherein a contacting range, in the third direction, of the unit and the contacting part is smaller than a contacting range, in the third direction, of the unit attached to the attachment part and the pressing plate.