A spark plug comprises: a tubular metal casing limiting axial direction; an insulator supported in the metal casing, wherein the insulator comprises a hole extended inwards along axial direction and a part of the insulator exposed out of the front end of the metal casing; and a central electrode inserted and fixed in the hole, wherein the point of the central electrode is exposed out of the frontend of the hole, and the center of the top is offset from the center of the hole when viewing in a plane vertical to the axial direction. The diameter t of the hole and the diameter s of the central electrode satisfy that t-s is larger than or equal to 0.05mm and smaller than or equal to 0.140mm, and the minimum clearance size x and the maximum clearance size y both limited between the peripheralsurface of the top of the central electrode and the inner surface of the hole satisfy that x/y is larger than or equal to 0 and smaller than or equal to 0.43.