Water treatment device and estimation method

This water treatment device is provided with: a measurement unit which repeats the measurement of the pressure on the primary side and the secondary side of a hollow fiber membrane module at a minute time interval of 3 seconds or less in one or more steps of water filling, backwashing, pressure relief, foaming, and water drainage; and an estimation unit that estimates the pressure on the basis of the lapse of time of at least one pressure measurement result or the difference between a plurality of steps on the basis of the lapse of time. The state inside the hollow fiber membrane module or the abnormality of peripheral devices, the time required from the start to the end of one or more steps, the effect of physical cleaning or chemical cleaning, or the trend of rising of the pressure difference between the membranes are estimated. 水处理装置具备：测量部，在充水、反洗、泄压、起泡及排水中的一个以上的工序中，每隔3秒以下的微小时间，重复进行中空纤维膜模块的一次侧及二次侧的压力的测量；以及推定部，基于至少一者的压力的测量结果的时间推移，或基于所述时间推移的多次工序间的差异，来推定中空纤维膜模块内部的状态或者周边装置的异常、一个以上的工序从开始到结束为止所需的时间、物理洗净或者药液洗净的效果、