[Problem] To provide as many heat exchanger tubes as possible and to make the gaps between the heat transfer tubes constant by devising a partition plate of a multitubular heat exchanger. [Solution] A manufacturing method of a multitubular heat exchanger (1) provided with multiple heat transfer tubes (2) for circulating a first fluid, a partition plate (3) provided at both ends of the heat transfer tubes (2), a chamber (40) enclosed by left and right partition plates (3), and a header (50) provided from the left and right partition plates (3) on the end side of the heat transfer tubes (2), wherein tubes (31), into which the heat transfer tubes (2) can be inserted, are bundled and cut, and multiple ring members (31a) are provided. Then, brazing material (32) is injected into gaps of said ring members (31a) and brazing is performed, and thereafter, the heat transfer tubes (2) are inserted into the ring members (31a) to fill in the gaps, forming the partition plate (3).
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