Abstract: An ion current detector device for an internal combustion engine, including: a spark plug having a metallic shell in which a tubular insulator is disposed, a center electrode extending in an axial direction of the tubular insulator and having a front end, a ground electrode which forms a spark gap (G1) with the front end of the center electrode, and at least one slanted bore provided in the metallic shell which is slanted inwardly against the axial direction with respect to the center electrode. An ion current detector electrode is provided within the slanted bore of the metallic shell and located proximate to the front end of the center electrode of the spark plug so as to form an ion current gap (G2) therebetween, the ion current gap (G2) being dimensionally wider than the spark gap (G1), and the spark plug being connected to an ignition circuit to be provided in a cylinder of the internal combustion engine.