To regulate the extrusion speed within an extrusion passage by a method wherein a plurality of resisting boards, which cross the inlet side of the extrusion passage and are different from one another in thickness, are detachably fixed through a plurality of liners different in thickness within a speed adjusting frame body.

A speed adjusting frame body 8 is made into a square frame body, at the central part of which a four-cornered center hole 8a opened on the inlet side of an extrusion passage. Each fitting recessed part 8b is formed between projecting parts 8c and 8c so constitute as to fit the end parts of resisting boards through liners 12 in this fitting recessed part 8b. The resisting boards 11 are spanned between the fitting recessed parts 8b and 8b opposing to each other so as to cross the center hole 8a and made into a plurality of boards different from one another in thickness. Then, a proper plurality of the resisting boards 11 different in thickness are selected in the state that the liners 12 and 12 are interposed respectively between the resisting boards 11 and 11.