A first fixed contact is mounted on an upper surface of a circuit substrate, and a second fixed contact is mounted on the upper surface of a circuit substrate around the first fixed contact. A movable contact having a semicircular sectional shape is mounted on the second fixed contact, and terminal electrodes are provided on an underside of the circuit substrate and connected to the first and second fixed contacts respectively. A spacer is mounted on the circuit substrate so as to surround the movable contact, and a cover is mounted on the spacer to cover the movable contact.