A water content detecting device for any type of diaper such as a cloth made reusable diaper, a paper made disposable diaper, or the like includes a sensing unit for sensing the degree of wetness of the diaper and an informing unit for transmitting the information representing the degree of wetness to a nursing person at a remote place. The sensing unit is detachably interposed in the diaper to be detected. The sensing unit is composed of a water impermeable upper sheet, a water permeable lower sheet, and a pair of metal layers placed between the upper sheet and the lower sheet and extending in the longitudinal direction of the sheets in parallel. One of the metal layers is covered with an electrical insulating layer. The informing unit is connected to the sensing unit through a pair of leads, and composed of a water content detecting circuit for detecting the change of electrostatic capacitance between the two metal layers of the sensing unit, a wave form shaping circuit, and an oscillating circuit. This water content detecting circuit mainly consists of an astable multivibrator which uses the two metal layers of the sensing unit as its feedback capacitor.