Abstract: The invention provides a water-tube boiler and a burner capable of realizing a reduction in NO.sub.x and CO in both the combustion of liquid fuel and the combustion of gaseous fuel. The water-tube boiler comprises a burner which uses liquid fuel or gaseous fuel switchably, characterized in that a plurality of water tubes are arranged into an annular shape in a zone where burning-reaction ongoing gas derived from the burner is present within a combustion chamber. The water-tube boiler comprises a burner which uses either liquid fuel or gaseous fuel. A first water tube array is formed by arranging a plurality of water tubes into an annular shape in a zone where burning-reaction ongoing gas derived from the burner is present within a combustion chamber. Gaps are provided between adjacent water tubes of the first ongoing gas to flow therethrough. A zone is provided around the first water tube array to allow burning reaction to be continuously effected.