To make it possible to obtain higher temperature measuring resolution even if a temperature sensor of what ever characteristics is used.

A resistance value R of a temperature sensor 2 when the central temperature within the intended temperature measuring range, e.g. 60°C is obtained. The value, which is obtained by subtracting the half of the voltage value of the variable signal outputted from a variable-voltage generating part 6 from the value that is obtained by multiplying a current value I of a constant- current power supply 1 and a gain G of an amplifier 3, is made to be the reference voltage outputted from a reference-voltage generating part 4.