Photocatalytic activity of powdered GaN for decomposing water into hydrogen under light irradiation was investigated. It was found that GaN has activity for producing hydrogen from water containing electron donors (Na2S-Na2SO3, CH3OH) without a loading of a noble metal; the obtained H-2 yield was ca. 150 mu mol for a 200 h-Xe lamp (300 W) irradiation. The activity was increased by adding NaOH in the reaction solution due to the photo-etching of GaN by NaOH and concomitant removal of the native oxide from the surface, as confirmed by XPS, XRD, and PL measurements. The band energy scheme for GaN suggests that the conduction band edge of GaN is positioned at 0.5 V higher than the redox potential of H+/H-2. This large energy difference (overpotential) makes the photocatalytic H-2 evolution over GaN from water possible.