To make a stable high speed forming for a long period of time possible by forming a tetra-guide which is a forming guide to open a folded film tube into a tetrahedron of which four surfaces are formed by an isosceles triangle, and providing rotatable rollers at a confronted areas with folding rollers.

A forming device for a film includes top and bottom two sets of rollers 2 which are arranged at a right angle, and in a film tube between the rollers, a guide of a tetrahedron (tetra-guide) 4 of which the four surfaces are formed by an isosceles triangle is inserted, and the position of a folding line which is newly formed is restricted. The tetra-guide 4 comprises a resin block of which the coefficient of friction is small, and on the two ridge sections being confronted with each other, rollers 5 are axially attached to an attaching fitting 6 so as to be rotatable in such a manner that the contact surface with the folding rollers 2 may slightly protrude from the surface of the tetrahedron. By this constitution, even in the case of a tube made of a material which is thin or has a low tensile strength, a forming can be performed without generating accidents such as a breakage.