To provide a low cost method and apparatus for isomerizing a gasoline having a high benzene content which is free from a secondary reaction.

This method for isomerizing a 5 or 6C hydrocarbon charge contg. mainly a paraffin hydrocarbon and 2 wt.% or more benzene comprises the step of passing the charge to be treated through at least one reactor 5 contg. a paraffin hydrocarbon isomerization catalyst at a total pressure equal to or greater than 10×105 Pa (10 bar) at an average temp. in the range of 100 to 200°C in the presence of hydrogen, wherein an additive, which is gaseous at 40°C and under atmospheric pressure (1.0134×105 Pa) and has a density equal to or lower than that of n-pentane under the same condition, is introduced into the upstream of a reaction region.