In this paper, temperature distribution of heated material (a frozen tuna) in a microwave oven is calculated by the method which combines FDTD for electromagnetic field calculation with heat transport equation considering melting heat and the nonlinear and rapid change of complex relative permittivity and heat conductivity during defrosting. As a result, the calculated temperature distribution of heated material agrees well with the measured one, and the validity of the combined method considering melting heat and the change of complex relative permittivity and heat conductivity is confirmed