A boiler comprising: a furnace having a hollow shape and installed along a vertical direction; a combustion device disposed in the furnace; a flue connected to an upper portion of the furnace in the vertical direction to cause flue gas to flow therethrough; an economizer (46,47) disposed in the flue; a reducing agent supplying device disposed in the flue on the further downstream side in a flow direction of the flue gas with respect to the economizer (46,47); a denitration catalyst disposed in the flue on the further downstream side in the flow direction of the flue gas with respect to the reducing agent supplying device; a bypass flow path having one end portion connected to the flue at a further upstream side in the flow direction of the flue gas with respect to the economizer (46,47), and a downstream side connection portion at the other end portion connected to the flue between the economizer (46,47) and the reducing agent supplying device; a first closing device which is provided in the bypass flow path on the further upstream side in the flow direction of the flue gas with respect to the downstream side connection portion with the flue, and partially closes the bypass flow path; and a second closing device which is provided in the flue on the further upstream side with respect to the downstream side connection portion of the bypass flow path with the flue and partially closes the flue, characterized in that the first closing device has a plurality of first closing members extending along the longitudinal width direction of the downstream side connection portion in the bypass flow path, along the flow direction of the flue gas in the flue and provided at predetermined intervals in a lateral width direction of the flue, the second closing device has a plurality of second closing members (65;71;72) extending in the flue along a vertical direction at the immediately upstream portion of the downstream side connection portion and provided at predetermined intervals in the width direction of the flue, the plurality of first closing members and the plurality of second closing members (65;71;72) are disposed to be shifted with respect to each other in the width direction of the flue and each of the first closing members has a plate shape, in which each end portion in a longitudinal direction is fixed to an inner wall surface of the bypass flow path and a planar surface on the downstream side is continuous with an inner wall surface of the flue without a step, and each of the second closing members (65;71;72) has a plate shape, in which each end portion in the vertical direction is fixed to the inner wall surface of the flue and a planar surface on the downstream side is continuous with the inner wall surface of the bypass flow path without a step. a furnace having a hollow shape and installed along a vertical direction; a combustion device disposed in the furnace; a flue connected to an upper portion of the furnace in the vertical direction to cause flue gas to flow therethrough; an economizer (46,47) disposed in the flue; a reducing agent supplying device disposed in the flue on the further downstream side in a flow direction of the flue gas with respect to the economizer (46,47); a denitration catalyst disposed in the flue on the further downstream side in the flow direction of the flue gas with respect to the reducing agent supplying device; a bypass flow path having one end portion connected to the flue at a further upstream side in the flow direction of the flue gas with respect to the economizer (46,47), and a downstream side connection portion at the other end portion connected to the flue between the economizer (46,47) and the reducing agent supplying device; a first closing device which is provided in the bypass flow path on the further upstream side in the flow direction of the flue gas with respect to the downstream side connection portion with the flue, and partially closes the bypass flow path; and a second closing device which is provided in the flue on the further upstream side with respect to the downstream side connection portion of the bypass flow path with the flue and partially closes the flue, characterized in that the first closing device has a plurality of first closing members extending along the longitudinal width direction of the downstream side connection portion in the bypass flow path, along the flow direction of the flue gas in the flue and provided at predetermined intervals in a lateral width direction of the flue, the second closing device has a plurality of second closing members (65;71;72) extending in the flue along a vertical direction at the immediately upstream portion of the downstream side connection portion and provided at predetermined intervals in the width direction of the flue, the plurality of first closing members and the plurality of second closing members (65;71;72) are disposed to be shifted with respect to each other in the width direction of the flue and each of the first closing members has a plate shape, in which each end portion in a longitudinal direction is fixed to an inner wall surface of the bypass flow path and a planar surface on the downstream side is continuous with an inner wall surface of the flue without a step, and each of the second closing members (65;71;72) has a plate shape, in which each end portion in the vertical direction is fixed to the inner wall surface of the flue and a planar surface on the downstream side is continuous with the inner wall surface of the bypass flow path without a step.