To provide as many as possible heat transfer pipes by devising partition plates of a multitubular heat exchanger, and to make gaps between the heat transfer pipes constant.

The multitubular heat exchanger 1 includes: the plurality of heat transfer pipes 2 for making a first fluid flow; the partition plates 3 provided on both end sides of the heat transfer pipes 2; an exchanger chamber 40 surrounded between the right and left partition plates 3; and headers 50 provided on end part sides of the heat transfer pipes 2 from the right and left partition plates 3. When producing the multitubular heat exchanger 1, pipes 31 through which the heat transfer pipes 2 can be inserted are bundled and cut, and a plurality of ring members 31a are provided. a brazing material 32 is injected into gaps of the ring members 31a, and is brazed, thereafter the heat transfer pipes 2 are inserted into the ring members 31a, and the gaps are filled to form the partition plates 3.