Liquid toner which remains on the surface of a photosensitive drum 2 after transfer of a toner image to an intermediate transfer member 1 is collected in a collection pot 6a. The collected liquid toner is led to a carrier pot 20, and is stored there. In response to an instruction, the stored liquid toner is fed to a toner concentration adjustment apparatus 10. Liquid toner which remains on the surface of a development roller 3 after supply of liquid toner from the development roller 3 to the photosensitive drum 2 is collected in a collection pot 6b. The collected liquid toner is fed directly to the toner concentration adjustment apparatus 10. Liquid toner having undergone concentration adjustment in the toner concentration adjustment apparatus 10 is fed to a supply pot 7, from which the liquid toner is supplied to the development roller 3 via a supply roller 4, whereby the liquid toner is used for development of electrostatic latent images. The toner concentration adjustment apparatus 10 includes a carrier extraction mechanism, a toner replenishment mechanism, a toner concentration detection mechanism, etc., which are incorporated in the concentration adjustment pot.