An ignition plug comprising: a tubular metallic shell; and an insulator having an axial hole extending therethrough in a direction of an axial line, provided internally of an inner circumference of the metallic shell, and having a rear trunk portion formed at a rear end portion thereof and protruding in an exposed condition from a rear end of the metallic shell, wherein the metallic shell and the insulator are fixed together by means of a crimped portion provided at a rear end portion of the metallic shell and bent radially inward, and the ignition plug is utilized for detecting ionic current, the ignition plug further comprising an electrically insulative filling member which fills a space formed between the crimped portion and the insulator; wherein the filling member covers at least a portion of an outer circumferential surface of the rear trunk portion along the entire circumference and the entirety of a rear end surface of the crimped portion, which surface is a portion of an outer surface of the crimped portion and is visible from a rear side with respect to the direction of the axial line; the ignition plug further comprising: a center electrode inserted into a forward end portion of the axial hole; and a ground electrode disposed at a forward end portion of the metallic shell and forming a gap in cooperation with a forward end portion of the center electrode, and characterised in that a length L along the axial line of that portion of the filling member which is located rearward of the crimped portion is greater than a dimension G of the gap. a tubular metallic shell; and an insulator having an axial hole extending therethrough in a direction of an axial line, provided internally of an inner circumference of the metallic shell, and having a rear trunk portion formed at a rear end portion thereof and protruding in an exposed condition from a rear end of the metallic shell, wherein the metallic shell and the insulator are fixed together by means of a crimped portion provided at a rear end portion of the metallic shell and bent radially inward, and the ignition plug is utilized for detecting ionic current, the ignition plug further comprising an electrically insulative filling member which fills a space formed between the crimped portion and the insulator; wherein the filling member covers at least a portion of an outer circumferential surface of the rear trunk portion along the entire circumference and the entirety of a rear end surface of the crimped portion, which surface is a portion of an outer surface of the crimped portion and is visible from a rear side with respect to the direction of the axial line; the ignition plug further comprising: a center electrode inserted into a forward end portion of the axial hole; and a ground electrode disposed at a forward end portion of the metallic shell and forming a gap in cooperation with a forward end portion of the center electrode, a center electrode inserted into a forward end portion of the axial hole; and a ground electrode disposed at a forward end portion of the metallic shell and forming a gap in cooperation with a forward end portion of the center electrode, and characterised in that a length L along the axial line of that portion of the filling member which is located rearward of the crimped portion is greater than a dimension G of the gap.