In an on-vehicle electronic control unit, error in output voltages from a constant voltage power supply circuit (C.V.P.S.) for use in a reference power supply of an A/D converter, occurs due to temperature rise. Whereby, there is a problem of the decrease of an A/D conversion accuracy. A C.V.P.S. supplies a voltage to analog sensors and to an AD converter. Furthermore, an output from a temperature sensor that is disposed in the neighborhood of the C.V.P.S. is stored in a memory as a contrast temperature data T at a time point of shipping. In addition, voltage fluctuation characteristic data due to the change of temperatures of the C.V.P.S. and detection fluctuation characteristic data accompanied by the change of voltages of the analog sensor group have preliminarily been stored, and the fluctuations in digital converted values accompanied with the change of temperatures in the neighborhood of the C.V.P.S. are compensated.