To provide a heat exchanger that eliminates or reduces the necessity of welding or brazing in a wide area of the case in assembling a case, and reduces manufacturing cost.

A case C housing a heat-transfer tube 1 and into which a medium for heating is internally supplied. The case C is a heat exchanger HE1 having a cylindrical case body 2 with an opening 25 formed in an end part, and side boards 6A, 6B for closing the opening 25. A periphery portion of the opening 25 of the case body 2 is formed as an extended part 26. A sealing member 7 is interposed between the extended part 26 and the side boards 6A, 6B, to fit the side boards 6A, 6B into the extended part 26.