Diffusion coefficients of tin under the existence of bismuth have been measured at 510° in the quasi-binary lead-tin systems. The result shows that diffusion coefficient in this system gives lower value than that of the same concentration in the binary system. This effect can be explained partly as due to the increase of activity coefficient of tin due to the addition of bismuth. This increase is estimated by the use of “interaction coefficient”, of which detailed expression has been already proposed by us.