To raise heat exchanging performance by ensuring brazing between a header pipe and a tube and preventing leakage of heating medium, in a heat exchanger consisting mainly of the heater pipe, the tube and a fin.

The header pipe and the tube are joined with a wax material by providing outwardly opening cone-shaped grooves to both the outside and the inside of a slit of the header pipe, into which the tube is to be inserted. An appropriate groove angle made by the groove surface of the header pipe and the surface of the tube is from 15 to 45 degrees. Preferably, an outer groove angle θ1 at the outside of the header pipe is rendered larger than an inner groove angle θ2 at the inside, and the tube surface should be provided with a wax material layer to attain a good brazing condition.