A deaerator and a density detector are provided in a liquid developer circulating path through which the liquid developer in a main tank is circulated, wherein the deaerator includes a bundle of hollow fibers each forming a hollow fiber membrane whose inside is depressurized and over whose outer peripheral surface the liquid developer is forced to flow under pressure, thereby sufficiently removing dissolved air in the liquid developer, and a toner density of the liquid developer is detected when the liquid developer of which dissolved air is sufficiently removed is passed through the density detector utilizing ultrasonic waves, making it possible to detect the toner density with a high degree of precision.