PURPOSE: To melt the outer periphery of the ends of tubes and weld the ends of the bundle of tubes homogeneously by fusion so as to have good sealing property and the shape of a honeycomb, by a method wherein the ends of a plurality of thermoplastic tubes are bundled and outer frame sleeves are fitted to the outer periphery of the ends, thereafter, the ends and the outer frame sleeves are heated by a heating plate.

CONSTITUTION: Both ends of a plurality of thermoplastic tubes 1, made of fluorine plastic, are bundled so as to obtain a column type bundle, and cylindrical outer frame sleeves 2a, 2b are fitted to the outer periphery of the ends of the bundle. The end face of the outer frame sleeve 2a is projected from the end face 1b of the tubes 1 and the whole of the bundle is set into a sealed vessel 3. The end face of the sleeve 2a, fitted to the ends 1a of the tubes 1, is abutted against a heating plate 4 to form the space 5 of gas between the end face 1a of the tubes 1 and the heating plate 4. The heating plate is heated for 40 minutes under the temperature of about 400°C. Adhesion between the outer periphery of the sleeve 2a and an expansion control surface 6 and the mutual adhesion of respective tube ends 1a are improved whereby the vessel 3 is sealed sufficiently, thereafter, air in the sealed vessel 3 is sucked to keep the degree of vacuum of 200W300 mmHg. Thereafter, the ends 1a of the tubes 1 are cooled by air. After the tubes 1 and their ends 1a are cooled perfectly, the bundled tubes 1 are taken out of the sealed vessel 3, then, the other ends of the tubes are welded by fusion.