To provide a water-cooled duct capable of maintaining constant cooling efficiency even with high-temperature exhaust gas and preventing damage to an inner cylinder by evaporated cooling water.

The water-cooled duct 1 comprises the inner cylinder and an outer cylinder, and includes a flowing water guide arranged between both the cylinders forming a flow passage for circulating and distributing cooling water delivered from a water supply header P from a water supply port 3a to a water discharge port 3b. The water-cooled duct 1 has a branch port 34 in the flow passage for branching the cooling water to the outside of the system, an auxiliary water supply port 35 for receiving the supply of the cooling water from the water supply header P separate from the water supply port 34, a temperature measuring instrument 36 for measuring the temperature of the cooling water branched from the branch port 34, and a control mechanism 37 for controlling the water supply amount to the auxiliary water supply port 35 according to a value of the temperature measuring instrument 36.

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