Toner replenishing device and image forming apparatus having the same

A toner replenishing device for replenishing a toner to a developer storing container for storing a two-component developer composed of a toner and a carrier which is disposed in a developing section where a toner image is formed by developing an electrostatic latent image formed on an image bearing member based on a multiple-value image composed of a plurality of pixels which are each represented by a gray scale value, comprising:

• (a) a replenishing section capable of replenishing predetermined first and second replenishing amounts of the toner to the developer storing container;
• (b) a toner consumption calculating unit for calculating a toner consumption in association with development of an electrostatic latent image corresponding to an inputted multiple-value image, based on the inputted multiple-value image, including:
• (b1) a gray scale value obtaining portion for obtaining gray scale values of pixels of the inputted multiple-value image;
• (b2) an edge detecting portion for detecting an edge in the multiple-value image;
• and
• (b3) a calculating portion for obtaining an estimated value of a toner consumption from the gray scale value obtained by the gray scale value obtaining portion, and a detection result of edge by the edge detecting portion, and calculating a cumulative toner consumption from a time when the first replenishing amount of the toner has been replenished to the developer storing container, with the use of the estimated value;
• (c) a concentration obtaining portion for obtaining a toner concentration of the two-component developer stored in the developer storing container;
• and
• (d) a control section for controlling the replenishing section so that when the cumulative toner consumption calculated by the toner consumption calculating unit reaches at least a predetermined reference consumption, the first replenishing amount of the toner is replenished to the developer storing container, and when the toner concentration obtained by the concentration obtaining portion falls below a predetermined start concentration, the second replenishing amount of the toner is started to be replenished to the developer storing container.