PURPOSE: To provide a filling method of a foaming substance which can prevent the generation of rust inside closed cross section structure members.

CONSTITUTION: A rust preventive is under-coated to a car body 10 which is made in a white body, and then, a foaming substance 26 is inserted in a closed cross section structure 24 from a foaming substance insertion hole 16 of a front pillar 12, and a foaming substance 26 is loaded to the loading part of a support plate 28. And a foaming substance 26 is inserted in a closed cross section structure 38 from a foaming substance insertion hole 18 of a center pillar 14, and the foaming substance 26 is inserted between the partition plates 44 in a closed cross section structure 42. Under such a condition, a second coating and a finish coating are applied to the car body 10, and the foaming substance 26 is foamed by the heating when the car body passes through the drying furnace of the second coating and the finish coating. Consequently, by the foaming substance in the closed cross section structures of the car body, a part where the rust preventive cannot be coated is eliminated.