To provide leaking-liquid sensor, a leaking-liquid detecting method and a liquid supplying and discharging system, which can perform the perfect detection of the leakage of water supplied into a device and equipment and can also detect leaking oil.

This leaking-liquid sensor has a water-supply channel 2, which supplies liquid circulating in a device and equipment 1, a water-draining channel 3 for discharging the circulated liquid, a bypass 4 for communicating the water supply channel and the water-draining channel through switch valves 5 and 6, flow-rate sensors 7 and 8, which are provided at the water supplying channel at the front stage of the directional control valves and the water draining channel, and a microcomputer controller 10, which performs the control of the switch valves, the detection of the detected value of the flow-rate sensor and the correction of the detected value of the flow rate.