The hydrogen gas production device includes a water tank in which a hydrogen-side photocatalyst is immersed in water in which a mediator is dispersed and hydrogen gas is generated by irradiation with light, and a mediator is oxidized; a light irradiation means for irradiating the hydrogen side photocatalyst with light; a water tank in which an oxygen-side photocatalyst is immersed in the water in which the mediator is dispersed and oxygen gas is generated by irradiation with light, and a light irradiation means for irradiating the oxygen side photocatalyst with light; and a water circulation unit for circulating water in which a mediator is dispersed between the hydrogen side and the oxygen side water tank, and a light source of the light irradiation means is a light emitting diode.