PURPOSE: To reduce production costs and to improve productivity by utilizing efficiently the heat generated in the course of injection molding for reducing the number of processes and for manufacturing preforms in the production of multilayered thin-walled containers.

CONSTITUTION: A sheet 22 is placed in a cavity R formed by a core 10, a lower mold 15, and a transfer mold 7 to mold a preform by injecting a resin into the cavity R. The preform is kept for a given time at a given temperature higher than the softening point of the resin by the core 10 and the lower mold 15, which are heated with band heaters 11, 16, and then transferred by the transfer mold 7 to the next processing stages such as vacuum forming to be molded into a container. It is unnecessary for the preform to be heated specially because it is kept at high temperatures by the core 10 and the lower mold 15.