A mean free path effect of impurities on a uniform flow in a superconducting alloy is investigated. By the use of the Gor'kov equation the change in the energy gap for a small current is given. Also the critical current is determined from the “de-pairing” condition. It is shown that the critical current vanishes as (l/ξ0)1/2, when l tends to zero, where l is the mean free path and ξ0 is the correlation length of the electron pairs.