Disclosed is a method for manufacturing carbon fibrous structures each of which comprises a three dimensional network of carbon fibers, which is characterized in that, in the procedure of carbon fiber vapor phase deposition utilizing decomposition of organic carbon compound, at least two kinds of organic carbon compounds as carbon sources are in existence at least at predetermined thermal decomposition reaction temperature range in a reaction furnace and partial pressures of gases of these compounds are regulated so that these compounds are decomposed at mutually different decomposition temperatures, and/or a raw material gas supplied into the reaction furnace is forced to form a turbulent flow. It is possible to obtain efficiently the carbon fibrous structures each of which comprises a three dimensional network of carbon fibers and which are suitable as additives to be added to solid materials, such as resin, ceramics, metal, etc., to improve physical properties, such as electric, mechanical, or thermal properties, of the solid materials, and also as additives to be added to liquid materials, such as fuels, lubricant oils, and etc., to improve physical properties, such as thermal property, of the liquid materials.