A polyimide membrane has a selective permeability for hydrogen gas and water vapor. A small scale experimental apparatus for reducing the tritium concentration of air was designed and fabricated at the Tritium Process Laboratory (TPL) in the Japan Atomic Energy Research Institute (JAERI). Hollow filament polyimide membranes are installed in a gas separation module (Ube Industries, Ltd.). Tritium tests were performed with air containing a small amount of tritium. Experimental results showed an increase of tritium concentration in the permeant flow. This method will be applied to tritium removal systems.