A ground-electrode spark portion 32 is formed from a noble metal which contains Pt as a main component, and is joined to a main metal portion of the ground electrode 4 via an alloy layer which has a thickness ranging from 0.5 .mu.m to 100 .mu.m and in which the noble metal that constitutes the ground-electrode spark portion 32 and the metal that constitutes the main metal portion of the ground electrode 4 are alloyed with each other. The ground-electrode spark portion 32 is configured such that a distal end surface 32t facing a spark discharge gap g is smaller in diameter than a bottom surface 32u joined to the ground electrode 4; and the distal end surface 32t is protrusively located beyond the side surface 4s of the ground electrode 4. When the ground-electrode spark portion 32 is viewed in plane from the distal end surface 32t, a portion of the surface of the ground-electrode spark portion 32 is viewed as a peripheral exposed-region surface 32p which is exposed on the side surface 4s of the ground electrode 4 so as to surround the distal end surface 32t.