A technical problem of this invention is to control the behavior of an intermediate layer in the preform bottom at high productivity and in a high degree of accuracy in the injection molding of a preform having a second-resin layer laminated with main-resin layers. A principal means of this invention to solve this technical problem is a process for injection molding a preform in a test tube shape having an intermediate layer of a second-resin layer laminated with main-resin layers. This process comprises the steps of: supplying a main resin from an outer flow channel and an inner flow channel to a joined flow channel at a predetermined supply velocity for a predetermined period of time; supplying a second resin from a middle flow channel to the joined flow channel simultaneously with the main resin at a predetermined supply velocity for a certain period of time within the predetermined period of time; and sliding a shut-off pin to bring the pin forefront to a predetermined position near an outlet of the inner flow channel, which is open to the joined flow channel, previously before the second resin is supplied, or during a period of time starting after a predetermined time from the start of the second resin supply and ending with the termination of the supply, so that the velocity of main resin supply from the inner flow channel to the joined flow channel is reduced to a predetermined level by adjusting the degree of aperture for the outlet.