An apparatus includes: a hydrogen generating agent (1) a hydrogen gas generator (3) having the hydrogen generating agent and at least one of a diluent gas supplier (7) or a vacuum pump (8) for diluting hydrogen gas generated by a reaction of the hydrogen generating agent and water (2) for reaction. The apparatus has a feature that a part or whole of a gas phase portion (4) in the hydrogen gas generator is covered with a hydrogen gas permeable membrane (5), and is operable to: mix the hydrogen gas with the diluent gas supplied from the diluent gas supplier or the vacuum pump thereby to supply a living organism with a mixed gas at a flow volume of 1 mL/min or more, the mixed gas having a hydrogen gas concentration of 0.1 vol% or higher and lower than 18.3 vol% and maintain a hydrogen gas concentration in a position separated by 7 cm from the hydrogen generating agent or a water surface of the water for reaction at lower than 18.3 vol% during hydrogen generation even when supply of the diluent gas is stopped.
