To provide a method and device for testing gear wherein the affect of a key way provided at a gear which is to be tested is reduced while the distortion of the gear is measured at higher precision than before.

A reference gear 12 and a gear 3 to be tested are urged using a spring 17 in such direction as they approach each other, for engagement with the reference gear 12 and the gear 3 rotated, the fluctuation in the center- to-center distance between the reference gear 12 and the gear 3 is represented in a function with periodicity using a calculation storage device 19. Here, a function is, for example, a sine function. Then the factor for the function is calculated by a method of least square using the center-to-center distance at a plurality of rotation angles. The amplitude of function calculated from the factor is recognized as an entire distortion amount of the gear 3. The measurement errors caused by play of a travel length measuring device and other device and that caused by a key way are almost cancelled by a method of least square, so the entire distortion amount 8 of the gear δ is measured at higher precision than with previous method.