A winding core holding mechanism is described for holding at least one end portion of a winding core on which a thin medium such as paper, film, or cloth is wound. The mechanism comprises a base fixed in the axial direction of the winding core. A larger diameter reference portion is included, which is capable of axially moving in and out of the base and abuts an end face of the winding core. The mechanism also comprises a tapered larger diameter centering portion which is capable of axially moving in and out of the larger diameter reference portion and fits into the winding core of larger diameter. Also included is a smaller diameter reference portion, which is capable of axially moving in and out of the base and abuts an end face of the winding core of smaller diameter. A tapered smaller diameter centering portion is further included, which is capable of axially moving in and out of the smaller diameter reference portion and fits into the winding core of smaller diameter. In order to hold the winding core of larger diameter, the larger diameter centering portion centers the winding core while falling into the larger diameter reference portion, and also the larger diameter reference portion falls into the base to position the end face at a predetermined reference position with respect to the base. In order to hold the winding core of smaller diameter, the smaller diameter centering portion centers the winding core while falling into the smaller diameter reference portion, and also the smaller diameter reference portion, the larger diameter reference portion, and the larger diameter centering portion fall into the base to position the end face at the reference position.