The technical problem to be solved is to change the thickness of the colored resin layer of the preform gradually in the upward or downward direction, by utilizing a tendency of gradual decrease in the thickness of the colored resin layer caused by the flow of the main resin inside the preform mold when the main resin and the colored resin are injected into the mold. This involves adjusting the injection pattern including the time of starting and ending the supply of the main resin and the colored resin, and pressure or velocity profiles, reducing the thickness of the colored resin layer gradually upstream or downstream, injection-molding the preform in which a color-gradated portion associated with the thickness of the colored resin layer has been formed, and biaxially drawing and blow molding this preform into a bottle having a color-gradated portion.