The output of a water temperature sensor is converted into a voltage signal by a first set of resistors. The voltage signal is then converted into a digital signal, which is supplied to a microcomputer for various controls. If the first set of resistors is so set that the voltage change provides optimum representation of temperature variations in a low-temperature range, the measuring accuracy in a high-temperature range becomes degraded. To improve the accuracy in the high-temperature range, the signal from the water temperature sensor is converted into a voltage signal through a second set of resistors--which is optimumly adjusted for the high-temperature range--by turning on a transistor. Thus, the sensor signal-voltage conversion means can be switched between the two conversion characteristics according to whether the temperature being measured is in the low- or high-temperature range, thereby providing high measuring accuracy in a wide range of temperatures.