To solve the problem that when a parison having an outer diameter thinner than an inner diameter of a bellows part is applied to prevent generation of a flash on the bellows part in a case of blow-molding a deformed and hollow duct with the bellows part, the molding itself is impossible since the resin of the parison can not go around the edge portion of the duct.

The problem is solved by pre-pinching the parison, the parison being shaped by a parison controller so that the position corresponding to the bellows part becomes necessarily sufficiently thick and the part other than the above becomes necessarily sufficiently thin, pre-blowing the parison by blowing air of relatively low pressure in the parison, then clamping the mold and shaping by blowing air of relatively high pressure.