To simply, certainly, and quickly manufacture an integrally-formed hollow article having a complicated shape by hot blow-forming.

Exhaust holes 3b, 4b communicating with the outside are formed in a male mold 3 and a female mold 4. A truncated cone part 3c having a recess part 3e communicating with a gas supplying hole 3f is formed in the male mold 3. The peripheral edges of an upper plate 5a and a lower plate 5b of a to-be-formed material 5 that are superimposed are sealed, and a gas suction hole 5d is formed in the central part of the upper plate 5a. In the mold 2, the recess part 3e of the truncated cone part 3c of the male mold 3 and the gas suction hole 5d of the upper plate 5a are brought into tight contact with each other so as to fit each other, and blow-forming is conducted while forming an enclosed space 6 by pinching and fixing the to-be-formed material 5. Since the to-be-formed material is expanded along the inner faces of the molds while exhausting the air held between the recess part of the die and the to-be-formed material to the outside, the integrally-formed hollow article is simply, certainly, and quickly manufactured.