To provide a gas heater for boiler flue gas processing equipment in which a heat transfer tube is effectually cleaned during operation of the equipment without provision of a shot cleaning apparatus and a soot blower, etc., and a heat exchange rate following an increase of pressure loss is prevented previously from being deteriorated.

The inside of a duct casing 12 is divided into a plurality of chambers 15 with a partition wall 14 in a widthwise direction perpendicular to a flow direction of combustion waste gas, and dampers 16 are provided at an inlet and an outlet of each chamber 15 defined with the partition wall 14. A cleaning apparatus is provided on an upper portion of each chamber 15 for injecting cleaning water to the heat transfer tube to clean the heat transfer tube 13 for each chamber in order.