To rapidly decelerate an engine without generating an engine stall by rapidly closing a throttle valve up to the specified opening, while gradually closing the throttle valve at the specified opening or less in the closing action of the throttle valve.

This accelerating/decelerating device for a carburetor is provided with a pressure chamber 26 communicating with an air bleed chamber 14 of the carburetor, and a diaphragm 25 for increasing/decreasing the volume of the pressure chamber 26. The diaphragm 25 is interlocked to decrease the volume of the pressure chamber 26 in the opening operation of a throttle valve 8 and to increase the volume of the pressure chamber 26 in the closing operation. A plurality of air passages 42, 43 provided with throttle parts 40, 41 are connected to the pressure chamber 26, and a valve mechanism 45 is provided for closing at least one air passage when the throttle valve 8 has a specified opening or less. The inflow of air into the pressure chamber 26 is thereby increased up to the specified opening to rapidly close the throttle valve 8, while the inflow of air into the pressure chamber 26 is reduced at the specified opening or less to gradually close the throttle valve 8.