To provide a boiler equipped with a water tube group performing heat recovery effectively and having expansion heating surfaces (fins or the like) of high durability, the present invention provides a boiler (1) including: a boiler body (10) having an inner water tube group (20) and an outer water tube group (30) that are arranged in an annular fashion; and a burner (40) arranged at a central portion of the inner water tube group (20), in which: intervals between adjacent inner water tubes constituting the inner water tube group (20) are closed except for portions where a gas flow passage is provided; and stud fins (22 and 32) are provided on a portion of at least one of the inner water tube group (20) and the outer water tube group (30) in a vicinity of the gas flow passage.