In an ion current detector device for an internal combustion engine, an ion voltage electrode is provided so that its front end is exposed to an inside of a cylinder or an exhaust passage of an internal combustion engine. An ion current power source is electrically connected to the ion voltage electrode. An ion current detector electrode is provided to form an ion current gap with the front end of the ion voltage electrode. A voltage detector circuit is provided between the ion current detector electrode and a resistor which is connected across the ion current detector electrode and the ground.