We propose a new method for determination of interdiffusion coefficient in high temperature solutions. This method can be characterized by the three following facts: the flux of diffusion is temporarily measured by in situ measurement technique for solution concentration in a diffusion cell; the intrinsic diffusion coefficient is derived from the flux on a basis of Fick's first law; the interdiffusion coefficients are evaluated from the intrinsic diffusion coefficients. In practice, the interdiffusion coefficient of GaZn binary system was successfully determined using the present method.