The present invention provides a method for treating wastewater and a device for treating wastewater for the same. A treatment method for parlor wastewater/biomass liquid digestate is provided with, in a cleaning facility provided with at least a mixing tank - reaction tank - sludge dehydrator - (neutralization tank) - biological treatment tank: a step for adding a dewatering aid having an Mp value (moisture content evaluation) of Mp (cake moisture content) ≤ 55 or 55 – 70 to parlor wastewater/biomass liquid digestate of water to be treated in the mixing tank; next, a step for adding a flocculant to the water to be treated in the reaction tank and forming floc that is maximum (10 mm ⌀ or greater) to large (5 – 10 mm ⌀) and that includes emulsified components and microparticle components in the floc containing the oil and fat portions and calcium in discarded milk or in the biomass liquid digestate; a step for eliminating the floc with the sludge dehydrator and simultaneously separating into the supe
