A spark plug has a spark plug housing of a cylindrical shape, an insulator of a cylindrical shape, a central electrode, a ground electrode and a terminal metal fitting. The insulator has a head part projecting toward a distal end side of the spark plug from the spark plug housing. The central electrode and the ground electrode forms a discharge gap. The terminal metal fitting, electrically connected to the central electrode, has a main body part and an exposed part. The exposed part of the terminal metal fitting has a head part. An outer diameter of the head part of the exposed part is greater than an outer diameter of the main body part. A ratio Dt/Di of not less than 0.8 is satisfied, Di represents an outer diameter of the head part of the insulator and Dt represents an outer diameter of the head part of the exposed part.