To provide a treatment method of wastewater from hydrothermal gasification which can purify wastewater discharged from the hydrothermal gasification process of organic waste.

A Mg component, such as Mg(OH)2, is added to the organic waste to perform the hydrothermal gasification. Wastewater containing large amounts of ammonium ions and phosphate ions are generated from the process, and air is bubbled into the wastewater to transfer carbon dioxide in the wastewater into air, which increases the pH of the wastewater to precipitate magnesium ammonium phosphate. Thereby the wastewater is purified, and at the same time, the magnesium ammonium phosphate, a valuable material, can be collected.