This invention relates to a liquid developer monitoring device comprising a first electrode which contacts with a liquid developer comprising a liquid medium and electrically charged toner particles dispersed therein, a second electrode being either a developing roller or a separate roller which provides a fresh surface and immerses said surface in the liquid developer, an electric power source which applies a bias voltage between said first and second electrodes so as to deposit the toner particles on the second electrode, and a sensor which measures magnitude of current flowing between said first and second electrodes during the deposition of the toner particles. A cleaning device removes the deposited toner on the second electrode. The sensor includes an electric coil, a magnet inserted in the coil and a Hall element.