An engine comprising: a cylinder head body provided with intake and exhaust ports opened toward a combustion chamber and capable of being closed by valves (60, 70), the intake and exhaust ports (40, 50) being separated from each other in a circumferential direction around a cylinder central axis; opening sections (41, 51) of the intake and exhaust ports facing the combustion chamber which are each provided with cylindrical surfaces (42, 52) which extend with the same inner diameter in a direction of a valve axis centered on the valve axis; valve seats (45, 55) each shaped as a ring and configured to be fitted onto the cylindrical surfaces (42, 52), in which inner circumferential surfaces of the valve seats (45, 55) become seat surfaces (46, 56) centered on the valve axis; tapered surfaces (43, 53) provided to the opening sections (41, 51) and disposed closer to the combustion chamber than the seat surfaces (46, 56), the tapered surfaces (43, 53) being gradually increased in diameter toward the combustion chamber, and a roof surface which faces a side of a cylinder block of the cylinder head body, and has a flat shape perpendicular to the cylinder central axis, wherein a central axis of the tapered surface is disposed eccentrically from the valve axis toward the cylinder central axis, and the engine further comprises the cylinder block having an inner wall surface which forms the combustion chamber along with a cylinder head, wherein inner diameters of open edges of the tapered surfaces (43, 53) which are intersecting ridgelines between the tapered surfaces (43, 53) and a roof surface of the cylinder head are set to be greater than inner diameters of the cylindrical surfaces (42, 52), wherein the roof surface of the cylinder head body abuts an end face of the cylinder block; and wherein the tapered surfaces (43, 53) are disposed to be present at a radial inner side of the inner wall surface of the cylinder block when viewed in the cylinder central axis of the engine; characterized in that in all of the intake and exhaust ports, the open edges of the tapered surfaces (43, 53) abut the cylindrical surfaces (42, 52) at most separated portions from the cylinder central axis when viewed in the cylinder central axis of the engine; and an antechamber cap is disposed in a center of the roof surface of the cylinder head body and provided with ejection holes through which flames are ejected into the combustion chamber, the flames being caused by ignition of an antechamber gas supplied to an inner hollow portion of the antechamber cap. a cylinder head body provided with intake and exhaust ports opened toward a combustion chamber and capable of being closed by valves (60, 70), the intake and exhaust ports (40, 50) being separated from each other in a circumferential direction around a cylinder central axis; opening sections (41, 51) of the intake and exhaust ports facing the combustion chamber which are each provided with cylindrical surfaces (42, 52) which extend with the same inner diameter in a direction of a valve axis centered on the valve axis; valve seats (45, 55) each shaped as a ring and configured to be fitted onto the cylindrical surfaces (42, 52), in which inner circumferential surfaces of the valve seats (45, 55) become seat surfaces (46, 56) centered on the valve axis; tapered surfaces (43, 53) provided to the opening sections (41, 51) and disposed closer to the combustion chamber than the seat surfaces (46, 56), the tapered surfaces (43, 53) being gradually increased in diameter toward the combustion chamber, and a roof surface which faces a side of a cylinder block of the cylinder head body, and has a flat shape perpendicular to the cylinder central axis, wherein a central axis of the tapered surface is disposed eccentrically from the valve axis toward the cylinder central axis, and the engine further comprises the cylinder block having an inner wall surface which forms the combustion chamber along with a cylinder head, wherein inner diameters of open edges of the tapered surfaces (43, 53) which are intersecting ridgelines between the tapered surfaces (43, 53) and a roof surface of the cylinder head are set to be greater than inner diameters of the cylindrical surfaces (42, 52), wherein the roof surface of the cylinder head body abuts an end face of the cylinder block; and wherein the tapered surfaces (43, 53) are disposed to be present at a radial inner side of the inner wall surface of the cylinder block when viewed in the cylinder central axis of the engine; characterized in that in all of the intake and exhaust ports, the open edges of the tapered surfaces (43, 53) abut the cylindrical surfaces (42, 52) at most separated portions from the cylinder central axis when viewed in the cylinder central axis of the engine; and an antechamber cap is disposed in a center of the roof surface of the cylinder head body and provided with ejection holes through which flames are ejected into the combustion chamber, the flames being caused by ignition of an antechamber gas supplied to an inner hollow portion of the antechamber cap.