A button key assembly of an operation panel includes button key units. Each button key unit includes button keys and a frame member. The button keys are arranged corresponding to switches on a substrate provided at the operation panel. The frame member supports the button keys via elastic arm members. The button key units are superposed one over the other to form the button key assembly. At least one of the button key units includes a supporting space having a width sufficient for supporting the button keys via the elastic arm members, and a vacant space disposed adjacent to the supporting space on a side closer to the elastic arm members and having a width narrower than the width of the supporting space. A button key of another button key unit superposed on the at least one button key unit is arranged in the vacant space.