Abstract: A water treatment device includes: a measure that repeats measurement of pressures on a primary side and a secondary side of a hollow fiber membrane module every minute time of three seconds or less in one or more steps of water filling, backwashing, pressure relieving, bubbling, and draining; and an estimater that estimates: abnormality of an internal state of the hollow fiber membrane module or of a peripheral device; a time required from a start to an end of the one or more steps; an effect of physical washing or chemical washing; or a future increase in an membrane differential pressure, based on a temporal transition of a measurement result of at least one of the pressures, or based on a difference in the temporal transition between a plurality of times of the step.