To acquire an error output highly accurately without a trouble concerning measurement/evaluation of tooth surface shapes of a pair of engaged gears, and to improve evaluation/management on measurement accuracy of the gears.

Each tooth surface which is a standard of measurement is treated as a conjugated tooth surface relative to the pair of the engaged gears 1a, 1b, and each error quantity is measured by using mutual contact points on the standard tooth surfaces as measuring points 5a, 5b, 6a, 6b. Each measurement error quantity of both gears is added together, to thereby calculate the relative error quantity of the pair of the gears 1a, 1b, and a relative tooth surface error shape is drawn by linking the relative error quantity of each of a series of measuring points 5a, 5b, 6a, 6b.