PURPOSE: To provide a low bulk density mat having a good resin-impregnating property and a good deep drawing property and suitable as a reinforcing material for foamable resins by dropping and accumulating many glass fiber bundles on a moving conveyor in a loop-like shape and subsequently mutually interlacing the glass fiber bundles for their incorporation.

CONSTITUTION: A plurality of glass fiber bundles produced by collecting many glass fibers with a collecting agent are dropped and accumulated on a moving conveyor in the loop-like state. The equipped with air-charging pipes opened in the passageway, and compressed air is blown on the glass fiber bundles to open the fibers. The treated fibers are accumulated on a moving conveyor in a loop-like state. Subsequently, a high speed air flow is passed through the loop-like accumulation material to open the glass fiber bundles and mutually interlace the opened fibers for forming the mat.