An image forming apparatus having a first operating mode and a second operating mode, where the second operating mode has an image forming velocity different from that in the first operating mode. The image forming apparatus includes a developing-agent containing portion, which contains a developing agent, an optical developing-agent amount detecting portion, which detects the amount of developing agent in the developing agent containing portion, a processing portion, which obtains the amount of developing agent based on data on the amount of developing agent detected by the developing-agent amount detecting portion, and a storing portion, which stores information for correcting data on the amount of developing agent detected by the developing-agent amount detecting portion in the second operating mode. The processing portion corrects the data on the amount of developing agent detected by the developing-agent amount detecting portion in the second operating mode based on the information stored in the storing portion, and obtains the amount of developing agent based on the corrected data.