PURPOSE: To reduce a cost of processing by sizing inorganic fiber such as rock wool with cation synthetic polymer sizing agent, forming it corresponding to an outlet shape, and integrally forming it.

CONSTITUTION: Material suspension is adjusted in a wet foaming tank 20, and the adjusted material suspension is fed to a manufacturing tank 20 by a pump 21. Material (g) dehydrated by a dehydration tank 22 is placed on a manufacturing die 12. Then, the die is removed, and dried as it is. Hot blast is blown from above by a drying oven 23, or it is sucked from below by a vacuum pump 24. Thus, a duct of a heat insulator is obtained. In this manner, its moisture absorption characteristic is suppressed, and its strength is enhanced. Cation synthetic polymer is used as binder, and its adding amount is 10% with respect to rock wool.