PURPOSE: To prevent the air-fuel ratio from becoming excessively dense by increasing the opening of a choke valve as compared with the case under a low load during acceleration at the engine cold start in a negative-pressure actuator- type choke valve.

CONSTITUTION: A chock valve 3 closed while an engine is stopped applies a negative pressure to a diaphragm 7 through a pipe 12 and a throttle 11 via the full explosion during the engine start and pulls a rod 4, which rotates the chock valve 3 in the open direction. At this time, since the engine temperature is low, a servo valve 21 is opened, a negative pressure switch 22 is turned on due to a strong negative pressure, a valve 23 is opened, and further, a valve 20 is opened due to the movement of the rod 4. Therefore, the negative pressure in a diaphragm chamber partially leaks into the atmosphere through the valve 20, pipe 13, valves 21, 23, thus setting the chock valve to a predetermined opening. During acceleration under this condition, the negative pressure switch 22 is turned off, and the leak of the negative pressure of the said diaphragm chamber is stopped, thereby the rod 4 is strongly pulled, and the choke 3 is kept at a slightly opened opening.