In order to conform a specific correction table to the characteristics of a liquid developer to be used, the specific correction table is created by calibrating a standard correction table stored in advance by using a viscosity property value of a liquid developer for calibration at a current temperature. A concentration adjustment is performed by comparing a measured viscosity property value with a target viscosity property value which is temperature-corrected based on the specific correction table. Further, the viscosity property value of a liquid developer is detected by measuring the current of a motor for stirring the liquid developer, where a no-load stirring current value is measured in the absence of a liquid developer, and one or both of a measured current value and target current value is corrected based thereon.