PURPOSE: To regulate the hole diameter and wall thickness of hollow yarn by providing an air blowoff hole at the tip of a mandrel with plural small holes opening in the radial direction and controlling the air pressure or air flow rate of air blown from the air blowoff hole.

CONSTITUTION: The air pressure or air flow rate of air blown from an air blowoff (flowing) hole (3b) by an air feeding means is controlled to a prescribed value and hollow yarn during forming is pressed against the inner circumferential surface of a die 1 with the pressure of air blown from plural small nozzle holes (3c), (3c)... at the outer peripheral part of a mandrel 3. Thereby, the hollow yarn is flattened.