An engine comprising a combustion chamber defined by a crown surface of a piston , an inner wall surface of a cylinder in which the piston is slidably accommodated, and a pentroof ceiling surface formed in a cylinder head , wherein an opening of an intake port configured to supply intake air to the combustion chamber and an opening of an exhaust port configured to discharge exhaust gas from the combustion chamber are formed in the ceiling surface , a side on which the intake port is disposed is an intake side, and a side on which the exhaust port is disposed is an exhaust side, an injector configured to inject fuel into the combustion chamber is disposed on the intake side of the combustion chamber , the crown surface includes: an exhaust-side bottom part ; an intake-side bottom part ;; an exhaust-side sloped surface rising toward a center part of the crown surface from the exhaust-side bottom part ; an intake-side sloped surface rising toward the center part of the crown surface from the intake-side bottom part ; and a flat surface provided continuously between an upper end of the exhaust-side sloped surface and an upper end of the intake-side sloped surface , and extending in a direction perpendicular to an axial direction of the cylinder in the center part of the crown surface , and the exhaust-side bottom part and/or the intake-side bottom part are flat surfaces which extend perpendicularly to the axial direction of the cylinder , characterized in that no dent or no cavity is formed in the flat surface , and a surface area (S1) of the flat surface is larger than a surface area (S2) of the exhaust-side sloped surface . an opening of an intake port configured to supply intake air to the combustion chamber and an opening of an exhaust port configured to discharge exhaust gas from the combustion chamber are formed in the ceiling surface , a side on which the intake port is disposed is an intake side, and a side on which the exhaust port is disposed is an exhaust side, an injector configured to inject fuel into the combustion chamber is disposed on the intake side of the combustion chamber , the crown surface includes: an exhaust-side bottom part ; an intake-side bottom part ;; an exhaust-side sloped surface rising toward a center part of the crown surface from the exhaust-side bottom part ; an intake-side sloped surface rising toward the center part of the crown surface from the intake-side bottom part ; and a flat surface provided continuously between an upper end of the exhaust-side sloped surface and an upper end of the intake-side sloped surface , and extending in a direction perpendicular to an axial direction of the cylinder in the center part of the crown surface , and an exhaust-side bottom part ; an intake-side bottom part ;; an exhaust-side sloped surface rising toward a center part of the crown surface from the exhaust-side bottom part ; an intake-side sloped surface rising toward the center part of the crown surface from the intake-side bottom part ; and a flat surface provided continuously between an upper end of the exhaust-side sloped surface and an upper end of the intake-side sloped surface , and extending in a direction perpendicular to an axial direction of the cylinder in the center part of the crown surface , and the exhaust-side bottom part and/or the intake-side bottom part are flat surfaces which extend perpendicularly to the axial direction of the cylinder , characterized in that no dent or no cavity is formed in the flat surface , and a surface area (S1) of the flat surface is larger than a surface area (S2) of the exhaust-side sloped surface . <EOS>