A mutlitubular heat exchanger capable of increasing heat exchange performance without increasing the heat transfer area and capable of solving the problem of a large decrease in heat exchange performance due to adhesion of dirt. A mutlitubular heat exchanger comprising a group of inner pipes (heat transfer tubes) through which a first fluid passes, and an outer tube (trunk) through which a second fluid passes, wherein a plurality of groups of heat transfer tubes are installed with their opposite ends held by inlet−side and outlet−side holder plates positioned on the first fluid inlet and outlet sides, respectively. The heat transfer pipe consists substantially of a heat transfer tube main body (114) alone, and one or both of the larger diameter side opposed wall surfaces of the heat transfer tube main body (114) is formed with a number of knot−like projections (240) disposed longitudinally at predetermined intervals. The projections (240) produce longitudinal vortexes in the high temperature gas to agitate the latter, with the result that the heat exchange efficiency is increased.
