PURPOSE: To obtain air bag foundation excellent in measure stability and storage performance by forming this foundation of a compound fiber of core-sheath type complex structure formed of polyester as a core element and polyamide as a sheath element, and using compound fiber filament of a specific characteristic as warp and/or woof.

CONSTITUTION: This compound fiber used as the foundation for an air bag is of core-sheath type complex structure formed of polyester, substantially composed of an ethylene-phthalate unit as a base, as a core element, and polymide as a sheath element; and the ratio of the polyester core element to the complex fiber is 20-90wt.%. Complex fiber filament, provided with such a characteristic as to have heat contractive stress (75°C)≤0.10g/d and the double refraction ratio of the sheath part ≤56×10-3, is used as warp and/or woof to obtain the desired air bag foundation.