To provide a water management device and a water management method which measure the number of bacteria at a real time, and more suitably perform water quality control.

The water management device (the water management method) includes: a bacteria sensor 5 (the-number-of bacteria measuring step) for irradiating water in a water tank 2 with light having a specified wavelength, and measuring the number of bacteria included in the water within the water tank 2 sequentially based on scattering light emitted from the water in the water tank 2; and a control part 7 (control step) for controlling the state of the water within the water tank based on the number of the bacteria measured by the bacteria sensor 5. Thus, the number of the bacteria can be measured at a real time, and water quality control can be more suitably performed.