PURPOSE: To prevent tar components from being condensed and liquefied as well as to prevent the occurrence of troubles due to tar components by cooling thermally decomposed gas down to the temperature slightly above a dew point temperature of tar components by means of a heat-exchanger, and also by increasing the temperature by means of a gas compressor.

CONSTITUTION: Solid fuel F is gasified by the reaction of thermal decomposition with gasifying agent which is composed of air A and steam S at a gasification furnace 1. And solid components C such as char and the like in thermally decomposed gas G which are produced in the gasifying furnace 1, are removed by a gas solid separator 2, and are utilized as a part of fuel for the gasifying furnace 1. In the plant as described above, a heat-exchanger 11 is newly provided in place of a former cleaning tower. And a flow control valve 13 is controlled by a temperature regulator 12 so as to control the temperature of thermally decomposed gas at an outlet port of the heat-exchanger 11 at the temperature which is set to one above a dew point temperature of tar components contained in thermally decomposed gas G. This allows the flow rate of coolant flowing within the heat-exchanger 11 to be controlled, and also prevents the occurrence of troubles due to tar components.