PURPOSE: To firmly connect a connection part and increase the strength of a connection structure.

CONSTITUTION: A pipe joining structure of a bicycle frame comprises a hub (connection part main body) 22 having three cylindrical extended parts 220, a lower pipe (tube member) 10, vertical pipe (tube member) 14, chain stay (tube member) 16 installed for each cylindrical extended part 220 so as to form a connection part, an inserting material 30 disposed inside each of these tube members and also extended to the inside of the tubes of the cylindrical extended parts 220 forming the tube members and connection part, and a coating material coating the hubs 22 and connection part. Then, the inserting material 30 comprises a sealing material 31 formed of composite material and a foaming material 32 enclosed in the sealing material 31, and the covering material comprises an outer layer material 40 and a foaming material 41 disposed between the outer layer material 40 and the connection part. In addition, the lower pipe 10 and others and cylindrical extended parts 220 are connected rigidly to each other by a force produced from the foaming materials 32 and 41 by heating.