Frame on which a plurality of keys are operably mounted has a continuous surface defining a channel-shaped concave section extending in a direction where the keys are arranged, and a body section in which the frame is mounted has a supporting surface that contacts opposite sides of an opening of the channel-shaped concave section of the frame. The opposite sides of the opening of the channel-shaped concave section of the frame are fixed to the supporting surface of the body section. Thus, a closed-section structure is formed by the channel-shaped concave section of the frame and the supporting surface of the body section, which constitutes a semi monocoque structure extending in the key-arranged direction. Such a structure can enhance bending rigidity and twisting rigidity relative to the key-arranged direction.