PURPOSE: To suitably dry a warp sheet by maintaining the humidity of a warp passage for drying the sized warp sheet with hot air in a prescribed range.

CONSTITUTION: Hot air generated with hot air generators 5, 13 is blown on and dries a sized warp sheet Y traveling in a warp passage 1 longitudinally disposed between a sizing device and a cylinder drying device and surrounded from vertical and lateral directions in a warp-sizing machine, the hot air being blown out from the hot air-blowing ports 6, 14 of hot air-circulating passages 4, 12 positioned on the inlet side 2 and the exit side 3 of the passage 1. While a part of the high humidity hot air containing the moisture evaporated with the drying is returned and reutilized from the suction ports 8, 16, 9, 17 of both the ceiling and the floor through ducts 10, 11, 18, 19, the other is exhausted through an exhaust passage 21, and the outside fresh air is supplied into the passage 1 and mixed with the hot air. In the process, the humidities of the high humidity hot air are continuously detected with a sensor 26. The detected humidity is compared with a preliminarily set humidity, and the exhaust volume of the high humidity hot air and the supply volume of the fresh air are controlled so as to reduce the difference between the detected and set humidities.