Amorphous TiO₂ doped with N was characterized by its photocatalytic activity under visible light irradiation. The amorphous N-doped TiO₂ was prepared by the sol-gel method through heat treatment at a low temperature. The photocatalyst showing activity in visible light despite heat treatment at low temperature can be applied to plastics and has excellent utility. The N-doped TiO₂ appeared amorphous when heat-treated at 130 °C. It was converted into an anatase-type N-doped TiO₂ when this was calcined at 500 °C. The photocatalyst showed photocatalytic activities in the photocatalytic decomposition of formaldehyde and methylene blue under visible light irradiation. The photocatalyst exhibited a higher rate of hydrogen production than that of TiO₂ in photocatalytic decomposition of water under liquid-phase plasma irradiation. The bandgap of the amorphous N-doped TiO₂ measured by investigation of optical properties was 2.4 eV. The lower bandgap induced the photocatalytic activities under visible light irradiation.