To provide a method for treating a phosphorus-containing organic waste liquid, by which an object to be treated is concentrated without generating scale to reduce the volume of the object to be treated, consequently miniaturization of a device, reduction in heat quantity in use and efficient decomposition of organic matter are enabled, and phosphorus can be separated through a simple operation and to provide a device for treating phosphorous-containing organic waste liquid.

The object to be treated, which is composed of the phosphorus- containing organic waste liquid, is crushed with a crusher 6, decarboxylated by a decarboxylation device 7, passed through a heat exchanger 12 from an object-to-be-treated vessel 1 after the addition of a seed crystal, evaporated and concentrated in a concentration device 2 to deposit phosphate compounds such as calcium phosphate, magnesium phosphate on the seed crystal to grow the crystal and the concentrated material 30 is treated by high oxidation decomposition in a decomposition device 4 to oxidize and decompose the organic matter and the reaction liquid is solid-liquid separated in a separation device 5 to recover the crystal, which is used for fertilizer, or the like.