A process for injection molding a preform in the shape of a test tube to be used in biaxial drawing and blow molding, which comprises: flowing molten synthetic resins through at least three layer-forming channels consisting of a cylindrical inner flow channel;, a middle flow channel, and a cylindrical outer flow channel;; flowing the resins through a confluence disposed downstream of these layer-forming channels and through a joined flow channel;; passing the resins through a gate disposed at a position opposed to center of bottom; and injecting the resins into mold to form a preform; and which also comprises: supplying the inner flow channel and the outer flow channel with a main resin for the preform from at least one loader at a predetermined time span, pressure, or velocity; supplying the middle flow channel with a colored resin from another loader at a predetermined pressure and/or velocity during a predetermined time within said time span; allowing the colored resin to join the main resin from the inner flow channel and the outer flow channel at a confluence in such a manner that the layer of colored resin is sandwiched between the two layers of the main resin; forming a multi-layered molten resin fluid having an intermediate layer of the colored resin between the layers of main resin within the joined flow channel over a certain span of time; adjusting injection pattern including the time of starting and ending the supply of colored resin, pressure and/or velocity profiles; and gradually reducing the thickness of this intermediate layer at an upstream point of the flow in direction down to a level near the bottom of the preform so as to form a color-gradated portion caused by the change in thickness. flowing molten synthetic resins through at least three layer-forming channels consisting of a cylindrical inner flow channel;, a middle flow channel, and a cylindrical outer flow channel;; flowing the resins through a confluence disposed downstream of these layer-forming channels and through a joined flow channel;; passing the resins through a gate disposed at a position opposed to center of bottom; and injecting the resins into mold to form a preform; and which also comprises: supplying the inner flow channel and the outer flow channel with a main resin for the preform from at least one loader at a predetermined time span, pressure, or velocity; supplying the middle flow channel with a colored resin from another loader at a predetermined pressure and/or velocity during a predetermined time within said time span; allowing the colored resin to join the main resin from the inner flow channel and the outer flow channel at a confluence in such a manner that the layer of colored resin is sandwiched between the two layers of the main resin; forming a multi-layered molten resin fluid having an intermediate layer of the colored resin between the layers of main resin within the joined flow channel over a certain span of time; adjusting injection pattern including the time of starting and ending the supply of colored resin, pressure and/or velocity profiles; and gradually reducing the thickness of this intermediate layer at an upstream point of the flow in direction down to a level near the bottom of the preform so as to form a color-gradated portion caused by the change in thickness. supplying the inner flow channel and the outer flow channel with a main resin for the preform from at least one loader at a predetermined time span, pressure, or velocity; supplying the middle flow channel with a colored resin from another loader at a predetermined pressure and/or velocity during a predetermined time within said time span; allowing the colored resin to join the main resin from the inner flow channel and the outer flow channel at a confluence in such a manner that the layer of colored resin is sandwiched between the two layers of the main resin; forming a multi-layered molten resin fluid having an intermediate layer of the colored resin between the layers of main resin within the joined flow channel over a certain span of time; adjusting injection pattern including the time of starting and ending the supply of colored resin, pressure and/or velocity profiles; and gradually reducing the thickness of this intermediate layer at an upstream point of the flow in direction down to a level near the bottom of the preform so as to form a color-gradated portion caused by the change in thickness.