A process for injection molding a preform in a shape of a test tube, comprising: flowing molten synthetic resins through at least three layer-forming channels (15, 16, 17) comprising a cylindrical inner flow channel, a middle flow channel, and a cylindrical outer flow channel; flowing the molten synthetic resins through a confluence disposed downstream of the at least three layer-forming channels (15, 16, 17) and through a joined flow channel; passing the molten synthetic resins through a gate disposed at a position opposed to a center of a bottom of a preform to be molded; and injecting the molten synthetic resins into a mold to form the preform; supplying each of the inner flow channel and the outer flow channel with a supply of a main resin for the preform from at least one first loader at a time span, pressure, or velocity; supplying the middle flow channel with a supply of a colored resin from a second loader at a pressure and/or velocity during a time at the latter part within the time span; allowing the colored resin to join the main resin from the inner flow channel and the main resin from the outer flow channel at the confluence in such a manner that a layer of the colored resin is sandwiched between two layers of the main resin; forming a multi-layered molten resin fluid having an intermediate layer of the colored resin between the two layers of the main resin within the joined flow channel over a certain span of time; adjusting an injection pattern including at least one of a time of starting and ending the supply of the colored resin, pressure and velocity profiles; and gradually reducing a thickness of the intermediate layer at an upstream point of flow toward an upper part of the preform, in which a neck exists, so as to form a color-gradated portion caused by a change in the thickness. flowing molten synthetic resins through at least three layer-forming channels (15, 16, 17) comprising a cylindrical inner flow channel, a middle flow channel, and a cylindrical outer flow channel; flowing the molten synthetic resins through a confluence disposed downstream of the at least three layer-forming channels (15, 16, 17) and through a joined flow channel; passing the molten synthetic resins through a gate disposed at a position opposed to a center of a bottom of a preform to be molded; and injecting the molten synthetic resins into a mold to form the preform; supplying each of the inner flow channel and the outer flow channel with a supply of a main resin for the preform from at least one first loader at a time span, pressure, or velocity; supplying the middle flow channel with a supply of a colored resin from a second loader at a pressure and/or velocity during a time at the latter part within the time span; allowing the colored resin to join the main resin from the inner flow channel and the main resin from the outer flow channel at the confluence in such a manner that a layer of the colored resin is sandwiched between two layers of the main resin; forming a multi-layered molten resin fluid having an intermediate layer of the colored resin between the two layers of the main resin within the joined flow channel over a certain span of time; adjusting an injection pattern including at least one of a time of starting and ending the supply of the colored resin, pressure and velocity profiles; and gradually reducing a thickness of the intermediate layer at an upstream point of flow toward an upper part of the preform, in which a neck exists, so as to form a color-gradated portion caused by a change in the thickness.