PURPOSE: To enable a reinforcing string to be formed without the deformation of a shape by winding spirally the string stock in half molten state on a forming shaft in the state where the core for forming is preliminarily wound spirally on said forming shaft.

CONSTITUTION: While the core 39 for forming of an endless shape is spirally wound on the upper surface of each forming shaft 14 so as to pass said upper surface, the half way position from the finishing end to the starting and of the winding of the core 39 is unfastened downward, and the unfastened part is spirally wound in between each auxiliary shaft 29 so as to pass the internal of said shafts. Then, a string stock 1 is formed so that the core 39 forms U-shape portion 1a and long and short side portions 1b, 1c. While each stock 1 is spirally wound, the U-shape portion 1a and the short side portion 1c of each stock 1 are integrally welded in piles on the long side portion 1b thereof, whereby the synthetic resin tube provided with a reinforcing string B is formed continuously. Next, on the top of the reinforcing string B, a cutter 44 is stuck, and cut grooves C are continuously formed, whereby the core 39 is drawn out from the inside of the reinforcing string B through the cut grooves C, and is returned to the end portion of winding start. Then, a half molten beltlike stack 2 is fed to the top of the reinforcing string B from the die 46 of an extrusion forming device 45 and is attached thereto, whereby the cut groove C is closed.