To provide an accelerating/decelerating device which can improve in a freedom degree in design and reduce in manufacturing cost while simplifying a passage in a main air system of a carburetor.

A switching valve device 40 comprises a sealed valve chamber 42, atmospheric opening path 43 opened to the valve chamber 42, air introducing path 44 opened to the valve chamber 42, control pressure path 46 opened to the valve chamber 42 through a valve seat hole 47, and a control valve 48 blocking the atmospheric opening path 43 when the valve seat hole 47 of the control pressure path 46 is opened and holding the atmospheric opening path 43 opened when the valve seat hole 47 is blocked. The air introducing path is connected to a wall chamber W, the control pressure path 46 is connected to a pump chamber 20 of a pump device 18, also a leak passage 50 facing toward the atmosphere from the control pressure path branches. The leak passage 50 arranges an orifice part 52 and a one-way control valve 51 permitting a flow of air toward the control pressure path from the leak passage 50 and impeding a flow of air toward the leak passage 50 from the control pressure path.