To provide a detecting method of eccentricity of a master gear and gear to be inspected that does not require previous measurement of the vibration of the master gear and does not require periodic vibration measurement of the master gear, and to provide an eccentricity detecting device used for this detecting method.

The gear ratio between the master gear and gear to be inspected is set substantially m/w (where, m2, w2, m+w=2n+1, m and w do not have a common factor, and m and w are natural numbers). The master gear and gear to be inspected mesh with each other by pressure-contact, the variation of the distance between the centers of both gears while the master gear rotates w times is recorded, and the eccentricity of the rotation centers of the master gear and the gear to be inspected is detected by dividing the record of the variation into two and calculating it.