A spark plug includes an insulator and a metal shell arranged around the insulator and having a thread portion formed with a thread diameter of M12. The insulator has an engagement portion held on a step portion of the metal shell via a plate packing and a leg portion located to leave a clearance between the leg portion and an inner circumferential surface of the metal shell. Assuming that: L (mm) is a distance from a front end of the plate packing toward the front; and A (mm) is a size of the clearance, a site of the clearance where the size A becomes 0.5 mm or smaller is located at or rear of a position of 2.0 mm from the front end toward the front; and a relationship of A≧L×0.2+0.2 (mm) is satisfied within a range of 3.0≦L≦4.0.