Developer is supplied to a first spray pipe disposed in a developing bath. The inside of the developing bath is provided with a heater. A liquid-flow generating unit comprises a liquid-flow generating head and a branch duct, which is connected to a branch pipe of the first spray pipe via a rubber hose. A part of the developer flowing in the first spray pipe is forwarded to the liquid-flow generating head. By a gap-forming ring, a gap is secured between an outer surface of the heater and an inner surface of the liquid-flow generating head so that the developer uniformly flows along the outer surface of the heater in a longitudinal direction thereof. A heater cover is attached to the liquid-flow generating head. An inner surface of the heater cover inclines in the longitudinal direction of the heater.