PURPOSE: To improve heat retaining property and heat insulating property and to improve a mechanical and a thermal strength by a method wherein an intermediate part partitioned by a partition wall is arranged in a thick base material.

CONSTITUTION: The interior of a partial chamber 3 is partitioned into upper and lower partial chambers 6 and 7 by a partition shelf 5. A hollow part 12 is formed in the partition shelf 5 to hold a heat insulation property. A large rib 14 for reinforcement is formed at the front peripheral edge part of the partition shelf 5 and by forming a cavity part 15 in the rib 14 for reinforcement, heat insulation property and heat retaining property are maintained at a high value. Further, a partition wall 13 to partition the hollow part is worked as a reinforcing material, and a mechanical strength is maintained at a high value. Though the thickness of a rib is increased, by forming a cavity part, a reinforcing effect is increased, the increase of a thick section part owing to resin is decreased and weight is decreased.