PURPOSE: To impart no effect to detected temp., by providing a differential amplifier consisting of two transistors to an apparatus and varying the comparing voltage applied to one input of the differential amplifier even if the voltage of a power source varies.

CONSTITUTION: The titled apparatus is constituted of a battery 1, a switch 2, a lamp 3, a capacitor 4, a thyristor 5, resistors 6, 12, 13, 15, 17, PNP transistors 14, 16 constituting a differential amplifier and a negative characteristic thermistor 18. When the voltage of the battery 1 changes, the middle point voltage necessary for turning the thyristor 5 ON changes but, at the same time, comparing voltage obtained by voltage dividing resistors 11, 13 also changes and temp. dependence when the thyristor 5 is turned ON is eliminated. Therefore, even if the voltage of the battery 1 varies, it can be accurately informed that the temp. of cooling water reached predetermined temp.