This paper presents and accurately estimating method for thermal deformation of machine tools. An exponential digital filter is used in this estimating method. In case of the thermal deformation arising from the rotation of main spindle, filter coefficient Fn is defined as a function which depends on the time constant and rotational speed. Besides the thermal deformation of machine structure arising from the change of room temperature is estimated in consideration of the time constant. Experimental results show efficiency of this proposed method.