Abstract: For a diaper for a baby, a toddler, an old man, or the like, a device for informing a mother of the baby or the toddler or a person who looks after the user of the diaper that the diaper is wet and a sensor for use in the device are provided. A wet annunciator is comprised of: a wet sensor that forms electrostatic capacity between a pair of electrodes 3 and 4 adhesively formed so as to be slightly spaced apart from each other, such that the electrostatic capacity in the vicinity of the electrodes varies when the diaper gets wet; an oscillating device 11 for applying an alternating signal to the electrodes 3 and 4 of the wet sensor; a detector 13 for picking up a change of the electrostatic capacity between the pair of electrodes 3 and 4; and an alerting device such as a vibrator 15, a buzzer or the like, operative according to an output signal of the detector 13.