To reduce the plant cost by installing a cooler body placed in a hollow pressure container, an outlet pipe through which a coal gasification gas descending in a space surrounded by the cooler body is sent out of the container, and a water-cooled pipe wall panel.

A coal gasification gas 8 at about 1,400-1,500°C generated in a coal gasification oven 1 is sent together with slag 30 through a gas sending channel 5 into a pressure container 12 and then is caused to descend in a space 28 of a cooler body 13 while being subjected to divided cooling by a water- cooled pipe wall panel 31. The gas 8 heats and evaporates supplied water ascending through a steam generating pipe 14 of the cooler body 13 and supplied water ascending through a water-cooled pipe 30 of the water-cooled pipe wall panel 31, thus supplying steam from outlet headers 24 and 36 to the outside. The gas 28, cooled to about 700-900°C while descending through the space 28 to a constricted part 17, is sent to a gas outlet pipe 29, and slag 30 separated from the gas 8 is caused to fall down into slag hopper water 27. Since the space 28 surrounded by the cooler body 13 is effectively used as a heat radiation zone, the pressure container 12 can be downsized.