The sorting method includes: an X-ray inspection step of irradiating a resin piece with X-rays including a first X-ray and a second X-ray having respective energy ranges different from each other and measuring a first transmission intensity which is an intensity of the first X-ray transmitted through the resin piece and a second transmission intensity which is an intensity of the second X-ray transmitted through the resin piece; a first determination step of making a determination as to whether the resin piece is a candidate for a useful resin piece, using the first transmission intensity; and a second determination step of making a determination as to whether a resin piece identified as a candidate for a useful resin piece in the first determination step is a useful resin piece, using a differential value obtained from the first transmission intensity and the second transmission intensity.