A water treatment method includes: introducing an aqueous solution containing impurities into a first tank; irradiating, from a light source, photocatalytic particles with ultraviolet light, to turn the aqueous solution introduced into the first tank into primary post-treatment water with which the impurities have been treated; reducing pressure in a first chamber by a filtering pump to filter the primary post-treatment water with a filtering membrane, the filtering separating the primary post-treatment water into the photocatalytic particles stopped at the filtering membrane and secondary post-treatment water that passes through the filtering membrane and does not contain the photocatalytic particles; measuring concentration of photocatalytic particles in the first tank; measuring the difference in pressure between the first chamber and the second chamber; and when the concentration of the photocatalytic particles is equal to or smaller than the first threshold value and the difference in pressure between the first chamber and the second chamber is greater than the second threshold value, supplying air to the filtering membrane by operating a compressor.