To a toner used in an image forming apparatus is added an external additive whose main components are oxide fine particles in which dimethylsilyl groups are introduced to surfaces thereof, and whose volatilizing amount of octamethylcyclotetrasiloxane in a first volatilizing test is 0.2 .mu.g or less. Consequently, it is possible to prevent foreign matters from attaching to a discharging electrode included in the image forming apparatus. The first volatilizing test is performed in such a manner that 2 g of the external additive is put in a sealing container and the inside of the sealing container is kept at 120.degree. C. for 10 minutes so that octamethylcyclotetrasiloxane volatilizes from the external additive.