Polymeric material fusion joining method, tube joint structure, and endoscope

A polymeric material fusion joining method, comprising:

• a first step of abutting a base tube and a mating member that are to be joined to each other so as to form a joint seam at an abutment junction thereof;
• a second step of fitting a tubular interlocking sleeve, which spans across the joint seam, over an outer periphery of the base tube and the mating member;
• wherein the interlocking sleeve is made of a polymeric material;
• and the method further comprises:
• a third step of heating the interlocking sleeve to a molten state to cause thermal contraction thereof, such that at least a portion of the material of the interlocking sleeve infiltrates into an entire circumferential wall of the base tube and the mating member that are not in a fully molten state, and fills the joint seam over an entire circumference thereof, so as to maintain smoothness of inner peripheral surfaces of the base tube and the mating member;
• and
• a fourth step of allowing the interlocking sleeve to cool and solidify, using only the interlocking sleeve as a connecting member so as to form an interlocking engagement with the base tube and the mating member, respectively, and to seal the joint seam.