The temperature field during the friction welding process is numerically estimated. The friction welding process of SUS304 stainless steels is analyzed using the finite element method. The heat coefficient A_l is proposed for the frictional heating: q_o=A_l(3Q)/(4πR^6)r. The A_l value is deduced from the experimentally obtained temperature history of several points on the specimen's surface. The heat input surface is translated with time evolution to reflect the discharge of the weld flush from the joining interface. To decide the A_l value and the translation length of the heat input surface, the frictional joining experiment is conducted for SUS304 stainless steel bars, and the temperature history and the compression length of the specimen are measured. The obtained A_l values are varied with time. By using the Ai values and the compression length as the translation length of the heat input surface, the precisely identical temperature distribution can be calculated.