Unbranched or less branched paraffins having 6 carbon atoms as essential components are isomerized without side reaction in high yield in the presence of hydrogen fluoride and boron trifluoride as a catalyst while keeping percent isomerization of paraffins having 6 carbon atoms lower than the equilibrium percent isomerization of the paraffins having 6 carbon atoms at a given reaction temperature, or by conducting isomerization at a plurality of stages while keeping a reaction temperature at a given stage lower than that at the preceding stage and keeping percent isomerization of paraffins having 6 carbon atoms at a given stage lower than the equilibrium percent isomerization thereof at the reaction temperature at the stage.