A spark plug (1) for an internal combustion engine includes: a cylindrical insulator (2) having an axial hole (4) penetrating in a direction of an axis (C1); a center electrode (5) partially inserted in the axial hole (4); a metal shell (3) surrounding an outer periphery of the insulator (2) and which is fixed to the insulator (2) by means of a crimped portion (20) provided at a rear end portion of the metal shell (3); and a ground electrode (27) joined to the metal shell (3) such that a portion of the ground electrode (27) opposes a leading end portion of the center electrode (5) via a spark discharge gap (33), wherein the insulator (2) has an annular groove portion (23) which opposes an inner edge of the crimped portion (20).