To easily remove ammonia in water by accelerating oxidation of nitrogen derived from ammonia by oxidizing ammonia in the water to be treated by a photocatalyst quickly and at the same time treating the produced nitrite ion and nitrate ion by ion removing means.

In the case this method for removing ammonia in water is employed for a water tank for aquarium fish, anion-exchange resin 1 is so arranged as to be immersed in water in the water tank and at the same time a photocatalytic reactor 3 is installed in the middle of a circulation route R formed as to circulate water of the water tank by a circulation pump P. The photocatalytic reactor 3 is constituted by filling a reactor main body 30 made of a quartz glass pipe with high ultraviolet transmittance with a large number of glass beads 31 bearing transparent titania coating on the surface while keeping gaps among the beads and installing four light sources 4, which are low pressure mercury lamps, emitting light of 180-190 nm wavelength in the surrounding of the reactor main body 30. Consequently, the ratio of ammonia remaining in water is significantly decreased to cause good effect on raising aquarium fish.