an intake control valve that is set in an intake path of an internal combustion engine and controls an intake air amount to be introduced into a cylinder by changing opening angle;

an EGR apparatus having an EGR path that connects an exhaust path of the engine to an intake path located in an intake downstream from the intake control valve and circulates a portion of exhaust gas through the cylinder as EGR gas, and an EGR control valve for controlling a flow rate of EGR gas that flows through the EGR path;

intake-pressure detecting means that detects intake pressure of the intake path located in the intake downstream from the intake control valve;

fuel cut means that restricts the opening angle of the intake control valve to a given opening angle and stops fuel supply into the cylinder when the engine is in a deceleration state;

engine-speed detecting means that detects engine speed;

reduction control means that reduces the opening angle restriction on the intake control valve according to the engine speed that is detected by the engine-speed detecting means so that the intake pressure of the intake path located in the intake downstream from the intake control valve is not lower than given pressure when the opening angle of the intake control valve is restricted to the given opening angle by the fuel cut means; and

EGR-malfunction diagnosing means that activates the EGR apparatus to circulate the EGR gas when the opening angle of the intake control valve is restricted to the given opening angle by the fuel cut means, and diagnoses a malfunction of the EGR apparatus according to whether or not the reduction control means is activated, based upon a change in intake pressure before and after the activation of the EGR apparatus, which is detected by the intake-pressure detecting means, and reference intake pressure that is determined in two ways, depending upon whether or not the reduction control means is activated.