A noise detection circuit includes an open and close switch connected onto a signal line for transmitting a detection signal of an extraneous noise, and the open and close switch is configured so as to be in an off state during a time period when the switching of the conduction state of a solid state switch is performed by a control circuit, whereas the open and close switch is configured so as to be in an on state during a time period when the switching of the conduction state of the solid state switch is not performed. As a result, an extraneous noise occurring irregularly can be detected.