PURPOSE: Zig-zag grooves are formed in the heat exchanger pipe inner surface to promote an agitation of coolant and to provide a performance of heat exchanging operation.

CONSTITUTION: Inner groove working device having a radially arranged rotary wheels 5 with a tread face 5a is passed into a heat exchanging pipe 3 with being displaced on a circumference of the rotary wheel holder 4, and the zig-zag groove 3a is formed in the inner surface of the heat exchanger pipe 3 by the tread face 5a of the rotary wheels 5. A plurality of rotary wheels 5 are made to form the zig-zag grooves 3 of which number corresponds to that of the rotary wheel 5. Thereby, an agitation is applied to the fluid flowing in the heat exchanger pipe to improve a heat exchanging performance.