A combustion condition detecting device detects combustion ions between opposing electrodes in a combustion chamber by ion current flowing between the electrodes in response to a.c. current voltage applied between the electrodes. A low pass filter for eliminating current components having high order frequencies of the a.c. voltage is disposed in a primary side of the transformer so that a waveform of the a.c. voltage approximates a sine wave. Thus, detection accuracy of the combustion ions is prevented from being affected by variation of capacitive current included in the current flowing between the electrodes, wherein the variation of the capacitive current is caused by distortion of the waveform of the a.c. voltage.