Abstract: Disclosed is a heat exchanger core in which a fluid is caused to flow through a pipe body rectangular in cross section so that the heat exchange is effected between the flowing fluid and a heat medium in contact with the outer walls of the pipe body. Within the pipe body, a plurality of projections and/or grooves for flowing the fluid at an inclined angle with respect to the direction of the flow of the fluid (the axis of the heat exchanger core) and the streams flow through the inclined passages, respectively, and then are reversed in direction. Such a flow pattern is repeated so that the efficiency of heat transfer by conduction can be improved without the noticeable increase in pressure loss.