PURPOSE: To connect pipes of the same diameter in polybutene-1 resin in waterproof manner and intensively by enlarging an end of one pipe of the same diameter in polybutene-1 resin and press-fitting an end of other pipe into the enlarged hollow part of the pipe.

CONSTITUTION: Ends of pipes A, A' of same diameter in polybutene-1 resin are inserted or engated in a heating and melting device B at the same time and then outer surface of the end of one pipe A is partially melted by a recessed heater a as well as the inner surface of the other pipe A' is partially melted by projected heater b and then the end of the pipe A' is totally heated by the recessed heater b surrounding this end to become soft so that enlargement can be made. After separating both pipes A, A' from the heating and melting device B, both pipes A, A' are press fitted in each other so that the outer surface of the melted pipe A and the inner surface of the melted pipe A' are soldered and connected in engaging manner each other and intensively.