A calculation method of tooth surface-errors equivalent to axis displacements caused by bending and torsional deformations of gear shafts is proposed. An axis layout, shaft shapes, gear dimensions and applied torques for a cylindrical gear drive are described into one data file. The developed program can calculate the equivalent tooth surface as a contour line diagram. Through the calculation examples, it was shown that the equivalent tooth surface is changed complexly by the axis layout, the rotational direction, etc. Finally, the effects of the tooth surface modification based on the equivalent tooth surface were investigated through the calculation of the contact stress at the meshing teeth and the transmission error under the loaded condition.