Provided is a key switch structure wherein a membrane sheet (7) having a movable contact point (7e) and a stationary contact point (7d) is provided between a housing (5), which supports a key top (1) so as to allow the key top to be pushed downward and a base plate (8). The movable contact point (7e) is brought into contact with the stationary contact point (7d) by a downward, push to the key top (1), so that a switch is closed. In the key switch structure, the inside of a device's main body is completely shielded from outside air, and water is completely prevented from entering the inside of the device's main body. A plate (6) provided with a plurality of attachment holes (6g) corresponding to fixing pins (5f) of the housing (5) is provided on the membrane sheet (7), so that the fixing pins (5f) are secured in the attachment holes (6g) of the plate (6).