By activating aluminum fine particles obtained by milling aluminum alloy in water, reactivity of the aluminum fine Particles at low temperature is improved. The activation comprises a thermal shock treatment wherein the aluminum fine particles are repeatedly heated and cooled down rapidly, and subsequently these activated aluminum fine particles are stored in a refrigerator. By reaction between the activated aluminum fine particles and water molecules, a large amount of hydrogen gas can be generated at room temperature, therefore hydrogen gas or fuel for a portable-type fuel cells can be produced in large quantities at room temperature in a short period of time.