PURPOSE: To enhance working efficiency while preventing the danger from the aspect of work by automatically supplying a nut for attaching a synthetic resin product from the rear surface side of a mold cavity and continuously performing the integral molding of a parison and the nut to automatically insert the nut in a predetermined cavity surface.

CONSTITUTION: Spoiler attaching nuts 5 are supplied to a hopper 42 so as to be arranged in the same direction to be inserted in a cavity surface 2 in a state protruding by desired length. A parison 12 is suspended from the lower end surface of a die 6 over predetermined length and the lower part of the parison 12 is pinched and compressed air of low pressure is introduced into the parison 12 from a pre-blowing pin 30 and, at the point of time when the lower end part of the suspended parison 12 passes the lower end parts of molds 1a, 1b, the extrusion of the parison 12 is stopped. In this state, both molds 1a, 1b are clamped and, when the parison 12 is cooled and solidified, the spoiler attaching nuts 5 provided to the cavity surface are also contained as a part of the parison 12 to obtain an integrated spoiler product.