For establishing a method to reduce thermally induced machining errors of machine tools, a method has been proposed to estimate the heat flux acting upon the machine tool body by solving inverse problems. Experiments were carried out with the cross slide of a machine tool. Using the estimated heat flux at the heat source, the temperature rise of the body in a steady state could be obtained with sufficient accuracy. More accurate estimation of the heat flux requires that the number and position of both temperature measurement points and candidates for heat source are selected considering the machine tool operation.