PURPOSE: To supply air required to maintain idling, without adding a special auxiliary valve by driving a second control valve that can supply air or mixture to the lower reaches of a throttle valve at the abnormal time of a first control valve, with the control quantity different from that at the normal time.

CONSTITUTION: In the case of being judged to be abnormal by a judging means 22, a canister purge flow control valve 8 is switched so as to be driven with the different control value from that at the normal time computed by a canister purge flow control valve driving quantity computing means 23. In the case of an idle state being judged by an idle state judging means 26, the output of a feedback control quantity computing means 24 for controlling the driving quantity so as to bring the engine speed 30 close to the target rotating speed on the basis of the difference between the target idling engine speed and the engine speed 30 is reflected on the canister purge flow control valve driving quantity. At the normal time, an operating area 27 set to stop in an idle operating state is made function even in the idle state. Air can thereby be supplied without adding a special auxiliary valve.