A spark-ignition internal combustion engine comprising: a cylinder; a piston disposed to be reciprocatively movable within the cylinder; a cylinder head disposed above the cylinder, and configured to form a pent-roof combustion chamber in cooperation with an inner peripheral surface of the cylinder and a top surface of the piston; a spark plug disposed in the cylinder head in such a way as to face the combustion chamber; an intake port and an exhaust port formed in the cylinder head in such a way as to open in a ceiling surface of the combustion chamber; an intake valve disposed in the cylinder head in such a way as to open and close the intake port; and an exhaust valve disposed in the cylinder head in such a way as to open and close the exhaust port, wherein a protrusion is formed on a top surface of the piston, the protrusion includes an intake-side inclined surface inclined along an intake-side ceiling surface of the combustion chamber and a valve head bottom surface of the intake valve, and an exhaust-side inclined surface inclined along an exhaust-side ceiling surface of the combustion chamber and a valve head bottom surface of the exhaust valve, a downwardly recessed cavity is formed in the protrusion at a position associated with the spark plug, and the intake port has a shape capable of generating a tumble flow within the combustion chamber, characterized in that the intake-side inclined surface and the exhaust-side inclined surface are formed in such a way that an angle (θ2) defined by an orthogonal plane orthogonal to a center axis of the cylinder and the exhaust-side inclined surface is smaller than an angle (θ4) defined by the orthogonal plane and the valve head bottom surface of the exhaust valve, and an inclination angle difference (θ4-θ2) between the exhaust-side inclined surface and the valve head bottom surface of the exhaust valve is larger than an inclination angle difference (θ3-θ1) between the intake-side inclined surface and the valve head bottom surface of the intake valve by 3 degrees or larger, and a downwardly recessed exhaust-valve recess is formed in the exhaust-side inclined surface at a position associated with the exhaust valve in such a way that a bottom surface of the exhaust-valve recess is parallel to the valve head bottom surface of the exhaust valve. a cylinder; a piston disposed to be reciprocatively movable within the cylinder; a cylinder head disposed above the cylinder, and configured to form a pent-roof combustion chamber in cooperation with an inner peripheral surface of the cylinder and a top surface of the piston; a spark plug disposed in the cylinder head in such a way as to face the combustion chamber; an intake port and an exhaust port formed in the cylinder head in such a way as to open in a ceiling surface of the combustion chamber; an intake valve disposed in the cylinder head in such a way as to open and close the intake port; and an exhaust valve disposed in the cylinder head in such a way as to open and close the exhaust port, wherein a protrusion is formed on a top surface of the piston, the protrusion includes an intake-side inclined surface inclined along an intake-side ceiling surface of the combustion chamber and a valve head bottom surface of the intake valve, and an exhaust-side inclined surface inclined along an exhaust-side ceiling surface of the combustion chamber and a valve head bottom surface of the exhaust valve, a downwardly recessed cavity is formed in the protrusion at a position associated with the spark plug, and the intake port has a shape capable of generating a tumble flow within the combustion chamber, characterized in that the intake-side inclined surface and the exhaust-side inclined surface are formed in such a way that an angle (θ2) defined by an orthogonal plane orthogonal to a center axis of the cylinder and the exhaust-side inclined surface is smaller than an angle (θ4) defined by the orthogonal plane and the valve head bottom surface of the exhaust valve, and an inclination angle difference (θ4-θ2) between the exhaust-side inclined surface and the valve head bottom surface of the exhaust valve is larger than an inclination angle difference (θ3-θ1) between the intake-side inclined surface and the valve head bottom surface of the intake valve by 3 degrees or larger, and a downwardly recessed exhaust-valve recess is formed in the exhaust-side inclined surface at a position associated with the exhaust valve in such a way that a bottom surface of the exhaust-valve recess is parallel to the valve head bottom surface of the exhaust valve.