To provide a water immersion warning system for a ship which gives an alarm according to water immersion level in order to prevent a foundering accident of a cargo ship as requested by International Maritime Organization for installation, where a sensor part can be checked even under existence of a cargo.

This system comprises a sensor means 2 provided with at least two electrodes 2a, 2b vertically in an insulator base 2c with a prescribed distance, a warning means 3 giving the alarm on receiving a detection signal from the electrodes 2a, 2b, and a checking means 4 checking whether the sensor means 2 connected with the electrodes 2a, 2b normally works or not.