The present invention utilizes jet streams of hot gas at a high efficiency and thereby to increase a bulk of non-woven fabric. Non-woven fabric containing thermoplastic synthetic fibers is subjected to jet streams of hot gas in a thickness direction of the non-woven fabric. A temperature of the jet streams of hot gas used for this process is set to a level lower than the temperature at which a resinous ingredient forming the surface of the thermoplastic begins to be melted. The jet streams of hot gas are directed to penetrate fiber interstices in the non-woven fabric and then to strike on a means serving to divert the jet streams of hot gas.