PURPOSE: To suppress the discharge of harmful gas by a simple construction by installing a pressure sensor for detecting the pressure in an intake pipe, a judging means for judging deceleration control, and a control means for adjusting an idle-speed control valve.

CONSTITUTION: An ISC valve 11 is installed into a bypass passage 10 which communicates with the upstream side and the downstream side of a throttle valve 7. A judging means 101 judges if an internal-combustion engine is in deceleration control or not, according to the detection result of a pressure sensor 9 for detecting the pressure in an intake pipe 5. When the judging means 101 judges that the internal-combustion engine is in deceleration control a control means 102 puts an idle-speed control valve 11 into a certain opening-degree state. Therefore, a sufficient amount of air can be supplied into the internal- combustion engine, and the fuel filling the inside of an intake manifold can be combusted perfectly. Thus, discharge of harmful gas can be suppressed with a simple construction in deceleration.