Water treatment device and operating method for water treatment device

A water treatment device is provided with a separation membrane device having a separation membrane for concentrating dissolved components and dispersed components from water to be treated and obtaining permeated water; a first deposit detecting unit provided in a non-permeated water branch line branched from a non-permeated water line for discharging non-permeated water in which dissolved components and dispersed components have been concentrated, using part of the non-permeated water that has branched off as a detection liquid, and having a first separation membrane for detection in which the detection liquid is separated into permeated water for detection and non-permeated water for detection; and first flow rate measuring devices for separated liquid for detection that measure the flow rates of one or both of the permeated water for detection and the non-permeated water for detection separated by the first separation membrane for detection.