An attachment structure for operation panel unit, comprising: an operation panel unit with a plurality of keys (20a - 20c) operated by a user; a main body to which the operation panel unit is attached; and an outer covering that is attached to the main body so as to cover the operation panel unit, the outer covering being configured to have a through hole portion in which covering-side key holes (13a - 13c) into which the keys (20a - 20c) can be inserted are provided, characterized in that: the operation panel unit is attached to the main body in such a way that the operation panel unit can slide with respect to the main body, in a plane perpendicular to a direction in which the outer covering is attached to the main body, the operation panel unit includes an operation panel frame to which the keys (20a - 20c) can be attached, first engaging portions (25a, 25c) formed as projections are provided on the operation panel frame around the foremost and rearmost keys (20a, 20c) which are included in the keys (20a - 20c), second engaging portions (15a, 15c) formed as projections are provided on the through hole portion of the outer covering, the second engaging portions (15a, 15c) being configured to correspond spatially to the first engaging portions (25a, 25c) when the outer covering covers the operation panel unit, and when the outer covering is attached to the main body in a state in which the operation panel unit is attached to the main body, the first engaging portions (25a, 25c) engage with the second engaging portions (15a, 15c), such that sliding of the operation panel unit is restricted by the outer covering. an operation panel unit with a plurality of keys (20a - 20c) operated by a user; a main body to which the operation panel unit is attached; and an outer covering that is attached to the main body so as to cover the operation panel unit, the outer covering being configured to have a through hole portion in which covering-side key holes (13a - 13c) into which the keys (20a - 20c) can be inserted are provided, characterized in that: the operation panel unit is attached to the main body in such a way that the operation panel unit can slide with respect to the main body, in a plane perpendicular to a direction in which the outer covering is attached to the main body, the operation panel unit includes an operation panel frame to which the keys (20a - 20c) can be attached, first engaging portions (25a, 25c) formed as projections are provided on the operation panel frame around the foremost and rearmost keys (20a, 20c) which are included in the keys (20a - 20c), second engaging portions (15a, 15c) formed as projections are provided on the through hole portion of the outer covering, the second engaging portions (15a, 15c) being configured to correspond spatially to the first engaging portions (25a, 25c) when the outer covering covers the operation panel unit, and when the outer covering is attached to the main body in a state in which the operation panel unit is attached to the main body, the first engaging portions (25a, 25c) engage with the second engaging portions (15a, 15c), such that sliding of the operation panel unit is restricted by the outer covering. the operation panel unit is attached to the main body in such a way that the operation panel unit can slide with respect to the main body, in a plane perpendicular to a direction in which the outer covering is attached to the main body, the operation panel unit includes an operation panel frame to which the keys (20a - 20c) can be attached, first engaging portions (25a, 25c) formed as projections are provided on the operation panel frame around the foremost and rearmost keys (20a, 20c) which are included in the keys (20a - 20c), second engaging portions (15a, 15c) formed as projections are provided on the through hole portion of the outer covering, the second engaging portions (15a, 15c) being configured to correspond spatially to the first engaging portions (25a, 25c) when the outer covering covers the operation panel unit, and when the outer covering is attached to the main body in a state in which the operation panel unit is attached to the main body, the first engaging portions (25a, 25c) engage with the second engaging portions (15a, 15c), such that sliding of the operation panel unit is restricted by the outer covering.