An internal combustion engine comprising: a cylinder block defining a cylinder; a piston slidable in the cylinder; a cylinder head mounted on the cylinder block to define a combustion chamber; an intake port and an exhaust port connected to the combustion chamber; an intake valve provided to the intake port to open and close the combustion chamber; an exhaust valve provided to the exhaust port to open and close the combustion chamber; a valve train (30, 50) for opening and closing the intake valve and the exhaust valve; a throttle body connected to an upstream end of the intake port; a throttle valve provided in the throttle body to open and close the throttle body; and a fuel injection valve for injecting fuel into the intake port; characterized in that: the valve train (30, 50) is of a fixed valve timing type keeping valve timings unchanged; and a relation expressed by Rv < 0.000438 × θ 2 - 0.0407 × θ + 1.55 is satisfied in a range of an intake valve overlap amount θ(°) from 20° to 45°, where Rv denotes an intake port volume ratio obtained by dividing an intake port internal volume Vp, as a volume of an intake port section from the throttle valve in engine idling to the intake valve in a closed state, by a cylinder stroke volume Vc as a total value of stroke volume of the cylinder which limits an intake air amount through the throttle valve; and where the intake valve overlap amount θ(°) is a crank angle from an intake valve opening timing when the intake valve starts opening to an intake top dead center of the piston; and a relation expressed by ηv ≤ Rv is satisfied to define a lower limit of the intake port volume ratio Rv, where the ηv denotes an idling volume efficiency obtained by dividing the intake air amount V 1 in an idling state of the engine by an exhaust gas amount V 0 as a stroke volume in the cylinder in the idling state of the engine, and Rv denotes the intake port volume ratio. a cylinder block defining a cylinder; a piston slidable in the cylinder; a cylinder head mounted on the cylinder block to define a combustion chamber; an intake port and an exhaust port connected to the combustion chamber; an intake valve provided to the intake port to open and close the combustion chamber; an exhaust valve provided to the exhaust port to open and close the combustion chamber; a valve train (30, 50) for opening and closing the intake valve and the exhaust valve; a throttle body connected to an upstream end of the intake port; a throttle valve provided in the throttle body to open and close the throttle body; and a fuel injection valve for injecting fuel into the intake port; characterized in that: the valve train (30, 50) is of a fixed valve timing type keeping valve timings unchanged; and a relation expressed by Rv < 0.000438 × θ 2 - 0.0407 × θ + 1.55 is satisfied in a range of an intake valve overlap amount θ(°) from 20° to 45°, where Rv denotes an intake port volume ratio obtained by dividing an intake port internal volume Vp, as a volume of an intake port section from the throttle valve in engine idling to the intake valve in a closed state, by a cylinder stroke volume Vc as a total value of stroke volume of the cylinder which limits an intake air amount through the throttle valve; and where the intake valve overlap amount θ(°) is a crank angle from an intake valve opening timing when the intake valve starts opening to an intake top dead center of the piston; and a relation expressed by ηv ≤ Rv is satisfied to define a lower limit of the intake port volume ratio Rv, where the ηv denotes an idling volume efficiency obtained by dividing the intake air amount V 1 in an idling state of the engine by an exhaust gas amount V 0 as a stroke volume in the cylinder in the idling state of the engine, and Rv denotes the intake port volume ratio. the valve train (30, 50) is of a fixed valve timing type keeping valve timings unchanged; and a relation expressed by Rv < 0.000438 × θ 2 - 0.0407 × θ + 1.55 is satisfied in a range of an intake valve overlap amount θ(°) from 20° to 45°, where Rv denotes an intake port volume ratio obtained by dividing an intake port internal volume Vp, as a volume of an intake port section from the throttle valve in engine idling to the intake valve in a closed state, by a cylinder stroke volume Vc as a total value of stroke volume of the cylinder which limits an intake air amount through the throttle valve; and where the intake valve overlap amount θ(°) is a crank angle from an intake valve opening timing when the intake valve starts opening to an intake top dead center of the piston; and a relation expressed by ηv ≤ Rv is satisfied to define a lower limit of the intake port volume ratio Rv, where the ηv denotes an idling volume efficiency obtained by dividing the intake air amount V 1 in an idling state of the engine by an exhaust gas amount V 0 as a stroke volume in the cylinder in the idling state of the engine, and Rv denotes the intake port volume ratio.