A method and device for estimating toner concentration in a two-component developer consisting of toner and carrier for an electrophotographic image forming apparatus, and an image forming apparatus equipped with such device. The above method and device calculate an amount of toner charge from data detected by a transferred toner amount sensor, compensate the toner charge based on environmental humidity data and cumulative use period data of the image forming apparatus, and estimate toner concentration. The image forming apparatus equipped with the above device exerts control to predict toner consumption from the estimated toner concentration, supply toner to a developing unit according to the predicted toner consumption, and maintain the toner concentration in the developer at predetermined concentration.