A material gas and a catalyst are introduced through a material supplying tube path and a catalyst supplying tube path together with a carrier gas into a reactor equipped on its outer periphery with a heat applicator for thermally decomposing the material gas. The reactor has a convention regulator fitted to the discharge end of the catalyst supplying tube path. The convection regulator covers an edge side of the reactor to regulate gas flow in the reactor so that the flow does not reach the edge side. Due to this, a convection state can be efficiently produced in a reaction region. Consequently, it becomes possible to prevent contamination defect caused by accumulation/adherence of concretion of catalyst, which was generated by aggregation of cooled catalyst in the low-temperature region of the reactor and a decomposition product of the material gas. Thus the efficiency of carbon nanostructure production can be improved.