An internal combustion engine provided with a plurality of cylinders, an intake valve and an exhaust valve provided to each of the cylinders, an independent intake passage branching for the respective cylinders and communicating at a downstream end thereof with each of the cylinders through the respective intake valve, and an independent exhaust passage branching for the respective cylinders and communicating at an upstream end thereof with each of the cylinders through the respective exhaust valve, the engine comprising: an intake camshaft including intake cam lobes configured to reciprocatably move the intake valves to have a given lift characteristic, respectively, and mechanically connected to the intake valves; an exhaust camshaft including exhaust cam lobes configured to reciprocatably move the exhaust valves to have a given lift characteristic, respectively, and mechanically connected to the exhaust valves; a variable phase mechanism (23, 24) configured to change rotational phases of the intake camshaft and the exhaust camshaft with respect to a crankshaft, respectively, so that a valve overlap during which both of the intake valve and the exhaust valve of the same cylinder are open is made, an injector configured to inject fuel into each of the cylinders; a spark plug configured to ignite a mixture gas containing fuel, air, and EGR gas inside each of the cylinders; a controller electrically connected to the injector and the spark plug, configured to control the injector and the spark plug by sending an electric signal; a water-cooled type EGR cooler; and an EGR valve disposed in an EGR passage, wherein the intake cam lobes are formed such that an open period of each intake valve from an open timing to a close timing is 210° or larger and 330° or smaller of a crank angle, wherein the exhaust cam lobes are formed such that, for each cylinder, during the overlap period when the variable phase mechanism (23, 24) advances the rotational phase of the intake camshaft to the maximum, and retards the rotational phase of the exhaust camshaft to the maximum, an amount of effective valve lift (Lift) of the exhaust valve, an inner circumferential length (L_ex) of a valve seat configured to contact the exhaust valve when the exhaust valve is closed, and a swept volume per cylinder satisfy the following formula, the amount of effective valve lift being a function of a crank angle from the open timing (CA IVO) of the intake valve to a middle timing (CA center) of the overlap period: 0.015 CA / mm ≤ L _ ex V × ∫ CA IVO CA center Lift CA dCA, wherein the controller is configured to control the injector and the spark plug so that, at least within part of an operation range of the engine, the mixture gas is ignited to start flame propagation combustion, and then unburned mixture gas is compressed to self-ignite, and wherein the controller is configured to control the EGR valve to adjust a flow rate of exhaust gas passing thought the EGR passage, wherein the controller is configured to control an amount of internal EGR gas to be increased as a load of the engine increases. an intake camshaft including intake cam lobes configured to reciprocatably move the intake valves to have a given lift characteristic, respectively, and mechanically connected to the intake valves; an exhaust camshaft including exhaust cam lobes configured to reciprocatably move the exhaust valves to have a given lift characteristic, respectively, and mechanically connected to the exhaust valves; a variable phase mechanism (23, 24) configured to change rotational phases of the intake camshaft and the exhaust camshaft with respect to a crankshaft, respectively, so that a valve overlap during which both of the intake valve and the exhaust valve of the same cylinder are open is made, an injector configured to inject fuel into each of the cylinders; a spark plug configured to ignite a mixture gas containing fuel, air, and EGR gas inside each of the cylinders; a controller electrically connected to the injector and the spark plug, configured to control the injector and the spark plug by sending an electric signal; a water-cooled type EGR cooler; and an EGR valve disposed in an EGR passage, wherein the intake cam lobes are formed such that an open period of each intake valve from an open timing to a close timing is 210° or larger and 330° or smaller of a crank angle, wherein the exhaust cam lobes are formed such that, for each cylinder, during the overlap period when the variable phase mechanism (23, 24) advances the rotational phase of the intake camshaft to the maximum, and retards the rotational phase of the exhaust camshaft to the maximum, an amount of effective valve lift (Lift) of the exhaust valve, an inner circumferential length (L_ex) of a valve seat configured to contact the exhaust valve when the exhaust valve is closed, and a swept volume per cylinder satisfy the following formula, the amount of effective valve lift being a function of a crank angle from the open timing (CA IVO) of the intake valve to a middle timing (CA center) of the overlap period: 0.015 CA / mm ≤ L _ ex V × ∫ CA IVO CA center Lift CA dCA, wherein the controller is configured to control the injector and the spark plug so that, at least within part of an operation range of the engine, the mixture gas is ignited to start flame propagation combustion, and then unburned mixture gas is compressed to self-ignite, and wherein the controller is configured to control the EGR valve to adjust a flow rate of exhaust gas passing thought the EGR passage, wherein the controller is configured to control an amount of internal EGR gas to be increased as a load of the engine increases.