Water treatment method and water treatment device

A water treatment method that uses a reaction tank and involves repeatedly performing an operation cycle including: an inflow step for causing an inflow of waste water; a biological treatment step for subjecting the waste water to biological treatment using biological sludge; a sedimentation step for causing the biological sludge to settle; and a drainage step for draining biologically-treated water that has undergone the aforementioned biological treatment. The reaction tank is provided with an inflow port that is disposed at a position lower than an interface position of a biological sludge bed formed at the bottom of the reaction tank in the sedimentation step, and an inflow pipe that extends upward in the vertical direction from the inflow port. In the inflow step, the waste water is caused to gravitationally flow down in the inflow pipe so as to be fed into the biological sludge bed from the inflow port.