When the remaining toner amount within a developer is relatively large, a control target value of a toner amount is set to Ttgt1, and a lower limit value of a toner amount which can regarded proper is set to TL1. When the density of a patch image formed under a predetermined condition is smaller than a minimum guaranteed density Dma, it is determined abnormality has occurred in the apparatus. Since the characteristic of toner within the developer changes as the remaining toner amount decreases, the control target value is changed to Ttgt2 and the lower limit value is set to TL2 in light of this. In this fashion, regardless of whether the toner characteristic has changed, abnormality in the apparatus is detected without fail.