A spark plug which satisfies .phi.B.ltoreq.0.8 mm, .phi.B/.phi.A.gtoreq.1.4, and 0.5 mm.ltoreq.C.ltoreq.1.2 mm, where .phi.A (mm) is the diameter of a distal end portion (51) formed of a noble metal tip (51b) of a center electrode 3; .phi.B (mm) is the diameter of a discharge portion (52) formed of a noble metal tip (52b) of a ground electrode (4); and C (mm) is a projection length of the discharge portion (52) from an electrode base metal surface (4a).