To prevent damage on an insulator due to thermal expansion difference between the insulator and a main metal fitting in a spark plug.

A spark plug includes a cylindrical insulator extending around the axis, and a cylindrical main metal fitting caulked to the outer peripheral surface of the insulator. Sealing powder is filling between the outer peripheral surface of the insulator and the inner peripheral surface of the main metal fitting. In the filling region where the powder is filling between the main metal fitting and the insulator, the minimum outer diameter d of the insulator and the maximum inner diameter D of the main metal fitting satisfy a relation 1.12≤D/d≤1.16.