PURPOSE: To prevent the power supply fluctuation which occurs in case of using the watch dog power supply of a controller, which processes the signal of a combustion pressure sensor, etc., and the power supply of the combustion pressure sensor, etc., in common.

CONSTITUTION: This is a power supply 44, which supplies a sensor 1 with constant voltage, and the sensor power supply fluctuation preventive device, which supplies constant voltage in common to a watch dog detecting the abnormality of a watch dog clear signal from a central processor 43 processing the output signal of the sensor 1 so as to control an engine but prevents the voltage fluctuation, is equipped with a WDC inversion inhibiter 60, which inhibits the inversion of the watch dog clear signal supplied to the watch dog, so this inhibits the inversion during the period when it converts the signal of the sensor 1 from analog to digital.