A thermistor with positive resistance-to-temperature characteristic used in a overcurrent protection circuit has electrodes on mutually opposite main surfaces and is mounted to a substrate having electrically conductive members such that deterioration of its voltage resistance due to heat emission can be controlled. A spacer with smaller thermal conductivity than the substrate and penetrated by a conductor piece with a small cross-sectional area is inserted between solder materials connecting to one of the thermistor electrodes. The other electrode is contacted by an elongated connecting member through its sectional surface transverse to its longitudinal direction such that the cross-sectional area of electrical conduction is reduced.