Provided is a reheat boiler in which the unbalanced temperature distribution of the gas temperature of a combustion gas on the outlet side of a reheat furnace is reduced by changing the gas flow pattern of a reheat burner. A reheat boiler (10A) is provided with a main boiler (106) which is configured in such a manner that a main combustion gas produced by the combustion of a burner (101) flows from a furnace (102) through a superheater (104) and a evaporation tube bank (105), a reheat furnace (108) which is disposed on the wake flow side of the evaporation tube bank (105) and produces a reheated combustion gas by the combustion of a reheat burner (107), and a reheater (109) which is disposed on the upper side of the reheat furnace (108), wherein a drift preventing portion is formed by providing a closing plate (130); for narrowing the cross-sectional area of the flow path of the combustion gas in a reheat furnace outlet portion (120) which forms the flow path of the combustion gas and the reheated combustion gas by connecting the reheat furnace (108) and the reheater (109).