A model equation for nuclide leaching from cementitious waste form is formulated based on diffusion, sorption equilibrium, and the continuity boundary conditions of concentration and flux on the waste form surface. Nuclide release unrelated to diffusive transport mechanism is reflected in the model as a wash-off fraction which depends on the surface conditions of the waste form. This leaching model is validated by comparison with the results of leaching experiments in various environments including land water, sea water and soils. It is expected that this leaching model will be used satisfactorily in analyzing various leaching experimental results.