A manufacturing method for a seamless hat that uses cloth material having a chemical fiber component, which is cut to form cut-out pieces having specific pattern outlines, then, after affixing superimposed edges of the stitched cut-out pieces, supersonic or high frequency equipment is used to administer heat processing on the fixedly joined edges of the cut-out pieces. Thereupon, any surplus material from the stitched cut-out pieces can be cut away, only leaving the very minimum of connecting gathered edges. Then, heating art is implemented to externally apply and cover the joined areas of the gathered edges with hold-down strips containing thermosol or thermal polyurethane (TPU) membrane, thereby fabricating a hat.