A malfunction determining unit 7 compares an absolute value of the difference between a first count and a second count stored in a storage unit 8 with a predetermined threshold. If the absolute value is less than a predetermined upper limit, the malfunction determining unit determines that a contact unit 1 has no malfunction. If the absolute value is the upper limit or more, the malfunction determining unit 7 determines that the contact unit 1 has malfunction, and supplies the determination result to a control unit 3. The control unit 3 allows an output unit 10 to output a malfunction detecting signal if receiving the determination result indicating the presence of malfunction from the malfunction determining unit 7.