An image forming apparatus having an improved developer density detecting device for detecting density of a liquid type developer without using an optical sensor. Density of the liquid type developer is detected based upon viscosity of the liquid type developer, which is obtained by sensing pressures, for example, of the liquid type developer sent by a pump and flowing through a developer circulating pipe and applying pressure data obtained by sensing pressures to a predetermined formula. The viscosity can also be obtained by dropping a steel ball, or moving a steel ball, through the liquid type developer and measuring properties based on movement of the steel ball. The viscosity can also be obtained by measuring torque of a device rotating with the liquid type developer.