There is provided an eccentricity amount estimation device, including: a rotation body provided with plural detected portions disposed along the rotation direction at predetermined rotation angle intervals, each of the plural detected portions being formed such that from an outer peripheral end towards an inner peripheral end a width of the detected portion at a position at the peripheral inside is the same as or greater than the width at a position at the peripheral outside; a generating section that generates a pulse signal according to passing of each of the plural detected portions accompanying rotation of the rotation body; and an estimation section that estimates a physical quantity corresponding to an eccentricity amount of the rotation body, based on a high level side pulse width and a low level side pulse width within one period of the pulse signal generated by the generating section.