A configuration for water purification undergoes the water supply step of taking in treatment water through a water suction port and pressure-feeding the treatment water, the air supply step of taking in air through an air suction port and supplying the air, the oxygen amount increasing step of pressurizing the air supplied at the air supply step to increase the total amount of oxygen, the ionization step of ionizing the pressurized air subjected to the oxygen amount increasing step, the mixing step of obtaining gas-liquid mixing fluid subjected to first fine air bubble formation by spraying the air into the flow of treatment water, and the accelerated spraying step of performing second fine air bubble formation by accelerating a flow velocity by a narrowed-diameter portion provided at a spray nozzle when the gas-liquid mixing fluid obtained through the mixing step is sprayed into the treatment water.