PURPOSE: To cool high temperature gas without coking on heat-transmitting faces and to recirculate separated solid particles by conducting the mixture of high temperature gas and solid particles along vertical group pipes with cooling medium running inside.

CONSTITUTION: Gaseout body of high temperature is blown in from a bottom entrance 8 of a cooling device main body 6 to by mixed with solid particles from a supply port 9 in the lower part of vertical group pipes 10, and the mixture is made to flow upward among the vertical group pipes 10 in a high speed. The gaseous body is deprived of its sensible heat by the cooling medium running inside vertical group pipes 10 to be rapidly cooled, and taken out from a line 11 to be introduced into a solidity separating means 12, where solid particles are separated to be recirculated to the supply port 9. Since ingredients of carbon and high boiling points are condensed and attached to the surface of solid particles in the gaseous body, the heat-transmitting faces of the group pipes are prevented from coking, and coking materials, even if given, are scaled off