A spark plug for an internal-combustion engine. The spark plug includes a central electrode, an insulator that holds the central electrode inside, a housing that holds the insulator inside, a ground electrode that forms a spark gap with the central electrode. The ground electrode includes a ground base member connected to the housing, and a ground protrusion protruding from a confronting face of the ground base member confronting the central electrode. The ground protrusion has a diameter of 0.9 to 1.4 mm. The spark plug is configured to satisfy θ≧−53R+49, where R [mm] is a curvature radius of a base member corner and θ[degrees] is an angle formed by a shortest connecting line between the base member corner and a protrusion corner and a plane perpendicular to an axial direction of the ground protrusion.