A spark plug includes a metal shell and a gasket. The metal shell includes a thread portion, a seat portion, and a thread root portion. The gasket has an inner diameter smaller than a thread size of the thread portion. The relationships 0.8L≦C≦L−2A and B≦0.75L are satisfied in a case where: in the seat portion, an outer diameter of a seating portion is L (mm); the maximum value of a relatively shiftable distance is A (mm), the gasket is shiftable along a direction orthogonal to the axis relative to the metal shell by the maximum value; in the gasket, an outer diameter is C (mm); and in the gasket, an inner diameter is B (mm).