PURPOSE: To enable abnormality of an idle revolution controller to be early judged on the basis of the change amount of the intake air amount in the idle stable state of an engine in the idle revolution controller for controlling the air amount to flow in the bypass passage of a throttle valve.

CONSTITUTION: In an idle revolution controller M1, the air amount to flow in a bypass passage M1B of a throttle valve M1A is controlled by a control valve M1C when an engine is idle-driven, so as to control the idle revolution to the target revolution. On the other hand, an intake air amount measuring means M2 measures the intake air amount to be supplied to the engine. Moreover a stable state detecting means M3 detects the idle stable state of the engine. And a change amount calculating means M4 calculates the measured intake air amount. And an abnormality judging means M5 judges abnormality of the idle revolution controller M1 in the detected idle-driving state when the change amount of the calculated intake air amount exceeds the specific value.