The rough-rice-drying process in a mixing type dryer was analysed by the theory of thin layer drying. The drying period of rough rice in the dryer is assumed to be a pre-heating period, and a heat balance equation is introduced as one of the basic equations. The Becker's equation of moisture content, the Henderson's equation of equilibrium moisture content, and the equations of air and vapour characteristics are used for the analysis with a slight modification. The drying process in mixing type dryer is assumed to be as folloes; 1） Drying air increase its humidity and reduces its temperature by absorbing the moisture of the products through the mixing layer from the inlet to the outlet. This means that the state of air changes with drying time and grain position as in deep stationary bed. 2) The state of grains is independent of their position, but dependent on drying time. This is due to constant mixing. 3) The drying rate of grains, averaged by constant mixing, is independent of their position although the local drying rate varies with their position. 4） The heating rate of grains is also independent of their position. This is the same situation as in the drying rate. The equations on the misture content, the temperature of grains, the absolute humidity, and the enthalpy of drying air are being derived. The numerical method of solving these equations is given together with its computer program. Results computed with this theory agree well with the measured values with the practical apparatus, L. S. U. type dryer.