Abstract: A method for rapid cooling of high temperature reaction gases obtained by ermal cracking of hydrocarbons, which comprises conducting an indirect heat exchange between a high temperature reaction gas obtained by thermal cracking of a hydrocarbon and a molten metal maintained at a prescribed temperature, lest heavy components in the high temperature gas be substantially condensed. In this method, the molten metal can be circulated by the use of a pressurized inert gas. In practice of this method, there can be effectively used a high temperature reaction gas cooling apparatus comprising a double-tubular heat exchanger, a cooling tank connected to said heat exchanger, and an inert gas circulating passage connected to a conduit connecting said heat exchanger and said cooling tank.