A multi-tube heat exchanger capable of increasing heat exchanging performance without increasing its heat transmitting area, as well as avoiding a reduction in heat exchanging efficiency due to adhering pollutant or the like. The multi-tube heat exchanger includes an inner tube group through which a first fluid passes and an outer tube around which a second fluid passes. The heat transmitting tube is constituted of only a heat transmitting tube main body and a number of lump-like projections formed at a predetermined interval in the length direction on one or both opposing wall faces on longer sides of the heat transmitting tube main body. Longitudinal eddies are generated in high-temperature gas by the, thereby consequently increasing heat exchanging efficiency.