To provide a boiler (boiler equipped with a rectangular water pipe group) for reducing the risk of overheating of a water pipe, and for obtaining steam with high dryness.

This boiler 1 includes an upper pipe gather 24, a lower pipe gather 25, the rectangular water pipe group constituted by vertically arranging a perpendicular water pipe group between the upper pipe gather 24 and the lower pipe gather 25, and a burner 10 disposed on a side surface of the rectangular water pipe group, and is characterized in that steam outlet pipes 81 and 82 communicating with the upper pipe gather 24 are provided, and baffle plates 91 and 92 are provided in the vicinity of openings of the steam outlet pipes 81 and 82.