A humidifier unit for use with a refrigerated display cabinet having a cooled air passage includes a body adapted to be mounted on an upper surface of the refrigerated display cabinet, a horizontally elongate atomizing water tank disposed in one side of the body and having a plurality of ultrasonic vibrators mounted on a bottom thereof and arrayed in a longitudinal direction, an air blower disposed in an opposite side of the body for supplying air drawn from a suction port into the atomizing water tank, and pipes angularly movably mounted on the atomizing water tank in coaction with the ultrasonic vibrators, respectively, for transferring a mist generated by the ultrasonic vibrators in the atomizing water tank and air from the air blower into the cooled air passage of the refrigerated display cabinet.