A ground electrode includes a main body portion and a projecting portion having a width smaller than a width of the main body portion. A spark discharge gap is formed between a discharging surface of the projecting portion and a front end surface of a center electrode. When the front end surface and the ground electrode are projected on a first plane, at least a part of the projection region of the projecting portion overlaps the projection region of the front end surface. A width Le (mm), a width Lc (mm), a cross-sectional area Sg (mm 2 ), a cross-sectional area Sc (mm 2 ), an angle θ1(°), an angle θ2(°), an angle θ3(°), and an angle θ4(°) satisfy expressions Le