A fuel cell is protected from performance degradation and damage by determining when an ion exchange resin filter needs replacement before the occurrence of an increase in the electric conductivity of water used in the fuel cell system. The distress signal can be an RF signal, which can be polarized. In order to increase efficiency of the incidence of light on the light receiver, the planar shape of the open part is formed so that the corners of the rectangle that surrounds the wiring structure are removed when the interlayer insulating film is etched and the open part is formed.