When the temperature of the liquid toner has fluctuated by T1 or more (Step S3: YES), the equation obtained by measuring the liquid toner is modified (Steps S4 through S7). This equation is obtained based on the output voltage value of the concentrated toner liquid sensor 27, which has the same specifications as those of the liquid toner sensor 24, and the output voltage value of the carrier liquid sensor 29, which has the same specifications as those of the liquid toner sensor 24 (S4 through S6). The output voltage value of the liquid toner sensor 24 is entered into the modified equation to calculate the toner solids content of the liquid toner of the storage tank 18 (Step S8). Control of the concentration of the liquid toner is carried out based on the calculated toner solids content (Step S9).