To provide a coasting richer of carburetor that can prevent air leakage when rapidly closing a valve, and reliably prevent occurrence of after-burning, in a two-cylinder engine equipped with one-body two-barrel carburetor.

In a coasting richer of a carburetor 1 that is a device provided in one-body two-barrel carburetor 1, is provided with two slow systems of air passages 8, 9 for each barrel, blocks one slow system of air passages 9 for each barrel when rapidly closing a throttle valve 6, and prevents occurrence of after-burning by making mixed gas to be temporarily rich, a one-way valve 22 for blocking the communication of the one slow system of air passages 9 with each other when rapidly closing the throttle valve 6 is provided in the one slow system of air passage 9 of the each barrel.