To provide a device for detecting the time when completing dewartering, for a dewatering machine, which contributes to efficient generation of dewatered cake having desired moisture content.

The device 10 for detecting the time when completing dewartering, for a dewatering machine, detects the time when dewatering is completed by the dewatering machine, by measuring total amount of filtrate from the start of dewatering of the dewatering machine 1. The device is provided with an ultrasonic flowmeter measuring the flow rate of the filtrate 12, a touch panel 18 inputting a target moisture content Wk of the dewatered cake as an input value, a computing means 19 computing a theoretical value A1 of the total amount of the filtrate discharged until dewatering completion by computing based on the input value, and a control means 20 sending dewatering completion signals to the dewatering machine 1, if the theoretical value A1 of the total amount of the filtrate is nearly equal to the total amount A2 of the actually measured filtrate.