1. ABSTRACTAn in-vehicle detecting apparatus has been developed which detects an ion current to monitor the combustion condition of each cylinder of the spark ignition internal combustion engine. This apparatus uses the spark plug electrode as an ion probe, and applies, as bias power, a very low ignition energy accumulated in the ignition coil. Thus it has the feature of requiring no particular modification on the engine side except a minor revision of the ignition system and addition of a detection module.This paper presents the construction and operation of this detection apparatus; in-engine evaluation and consideration on misfire and knocking level detecting performance; and consideration on effects of various factors on ion current values detected. In addition, data on the possibility of detecting an air-fuel ratio is reported.