PURPOSE: To obtain a long heat insulating pipe simply and promptly by a method wherein a foamed heat insulating material in the form of a pipe is cut and opened in the 1st hollow guide in the axial direction, advanced along the outside of the 2nd hollow guide under the cut and opened condition, put on a metal pipe that has passed through the inside of the guide and united into one body and the cut and opened part is welded.

CONSTITUTION: Foamed heat insulating material 8 in the form of a pipe is passed through the 1st hollow guide 1, cut and opened by cutters 2, 2 equipped in this guide 1 in the axial direction. The cut and opened part 8a of the foamed heat insulating material 8 that has passed through the guide 1 is expanded by a gradual diameter expander 3, then, advanced along the outside of the 2nd hollow guide 5 as this cut and opened state is kept and united into one body with a metallic pipe 9, that has passed through the inside of the 2nd hollow guide 5, through a horizontal guide roll 6 the instant they come in contact with each other. Then, the cut and opened part 8a of the formed heat insulating material 8 is welded by hot air, hot platens, etc. to obtain the desired heat insulating pipe.