A water treatment device (1) is provided with: a biological treatment tank (2) for storing a stored liquid (5); a pump (7) for generating a circulating liquid flow formed of the stored liquid (5); a gas-liquid two-phase flow generation device (8) for using the circulating liquid flow to suction a gas containing oxygen, whereby a gas-liquid two-phase flow in which the gas is dispersed in the circulating liquid flow is generated; and a nozzle (12) for injecting the gas-liquid two-phase flow into the biological treatment tank (2). Such a water treatment device (1) is capable of causing the stored liquid (5) to adequately circulate in the biological treatment tank (2), adequately aerate the stored liquid (5), and adequately treat the stored liquid (5).