The drying process of the surface water of the fresh farm products after the washing belongs to the preheating period for the quick drying to prevent a temperature rising of the product for keeping freshness, and for increasing efficiency. Therefore the drying rate can not be estimated by the formula of constant rate period, and the falling rate period. The process is transient simultaneous mass and heat transfer phenomena, and the complication and difficulty are increased by the existence of the temperature gradient in the product. For the purpose of dealing with this problem the equation of conduction of heat in semi-infinite solid with the boundary condition expressing the fact that the summation of the heat of the evaporation and the heat of the conduction into the solid is equal to the heat transferred to the surface by convection from the heated air, which is expressed in Eq. (4) and Eq. (5), is proposed in this paper. The equation is solved by Runge-Kutta method assuming the effective thickness which is determined by the analytical solution of the equation of semi-infinite body with the boundary condition of the heat convection only between the surface and the air. The experiment is performed using an egg on the surface temperature and the drying rate of the surface water. The surface temperature is measured by C-C thermocouples with a three pen recorder, and the drying rate is measured as the dry up time of the four divided parts on the surface with the eye because the mass of the water is so little comparing with the mass of the product that precise continuous measurement of the change of the mass of the water is very difficult. The empirical equation of the heat transfer of the egg is determined from the cooling curves of the lead model of an egg. The theoretical results on temperature agree well with the experimental results, and on drying up time agree well with the experimental results, too. It may be concluded that the theory is useful for the estimation of the drying rate and the temperature rise of the drying surface of the wet fresh farm products after the washing with the object of designing and operating the dryer.