A spark plug for an internal combustion engine that can improve bonding strength between a center electrode and a conductive glass is provided. The spark plug includes a housing, an insulator, a center electrode, a ground electrode, and a conductive glass. The center electrode has a locking portion that is locked from a base end side to a step portion formed on an inner peripheral surface of the insulator, and an electrode head that is closer to the base end side than the locking portion is. A concave portion is partially formed on the tip end surface of the electrode head. A concave contour, which is an outer peripheral contour of the concave portion when viewed in a plug axial direction, forms a closed curve which is spaced apart from a head contour, which is an outer peripheral contour of the base end surface of the electrode head, and surrounds a center axis (B) of the center electrode. The concave contour has an outward portion protruding toward the head contour and an inward portion protruding toward the center axis (B) of the center electrode.