A spark plug for an internal combustion engine is constituted by an insulating member (20) having an axial hole (26) formed along a central shaft (18); a rod-shape central electrode (12) accommodated in the axial hole; and a ground electrode (16) with which an igniting portion (16a) opposite to the outer surface (12a) of the central electrode is formed. In the spark plug, application of voltage between the central electrode and the ground electrode such that the polarity of the central electrode is made to be positive causes discharge to occur between the central electrode and the ground electrode. The spark plug satisfies at least one of the following conditions (a) and (b). (a) When the insulating member is cut along the central axis and a first extension line (60a) in the form obtained by outwards extending a line (60) indicating an end surface of said insulating member adjacent to the igniting portion and a second extension line (61a, 63a) in the form obtained by extending a line (61, 63) indicating the outer surface of the insulating member in the vicinity of the igniting portion are drawn, the distance tp from an intersection (62) between the first and second extension lines to a line indicating an inner surface of the axial hole adjacent to the igniting portion is 1.0 mm or shorter (tp ≦1.0 mm). (b) The difference (&phgr;d2−&phgr;d1) between diameter &phgr;d1 of the central electrode and diameter &phgr;d2 of the axial hole is 0.08 mm or shorter {(&phgr;d2−&phgr;d1) ≦0.8 mm}.