In an image forming apparatus having a toner container, the remaining amount of toner is detected by measuring a capacitance caused by a toner detector member in the toner container. It is determined whether the measured capacitance falls within a predetermined range. If it is determined that the measured capacitance falls not within the predetermined range, the measured capacitance is not used in the detection of the remaining amount of toner. The present invention thus provides an image forming apparatus reliable in the detection of toner, and a method of controlling the apparatus.