By storing a liquid to be treated in a treatment tank main body 10 having a bottom portion 10g that narrows downward, with particles of materials to be treated dispersed therein, taking out the liquid to be treated from a position lower than the liquid level of the treatment tank main body 10, and returning the liquid to the bottom portion 10g of the treatment tank main body 10, thereby circulating the liquid to be treated while forming a vortex flow within the treatment tank main body 10, and at the same time supplying air bubbles from the lower part of the treatment main unit 10, a first component contained in froths is separated from a second component, which is less susceptible to floatation, contained in the liquid to be treated.