A spark plug has a housing, an insulator, a central electrode and a ground electrode. A projection part is formed projecting radially from the housing. The spark plug has a structure which satisfies a relationship of t2/t1≤0.85, s1≤0.5, and s1≥1.05−t2/t1. A leg part of the insulator has a first end and a second end in an axial direction thereof. The second end is located opposite to the first end in the axial direction. In the relationship, t1 represents a first radial thickness of the first end in a radial direction of the leg part, t2 represents a center radial thickness in the radial direction at a middle position of the leg part, and s1 represents a first gap width in the radial direction of the housing between the projection part of the housing and the first end of the leg part.