set up so that a command can be transmitted from a host computer or the like connected to the image forming apparatus to switch the process speed it has been known that changing the process speed of an image forming apparatus affects the properties of the image which the apparatus forms more specifically the line width which reflects the amount of the cartridge usage is affected by the process speed fig3 shows the relationship between the width of a line four dots u00d7 six spaces formed at 600 dpi and the process speed process speeds va and vb it is evident from this graph that the higher the process speed the narrower the line width what is presumed as one of the causes of this phenomenon is that the higher the process speed the shorter the time it takes for a latent image on a photoconductive drum to pass through the development nip region in which distance between peripheral surfaces of photoconductive drum and development sleeve is smallest and therefore the smaller the amount by which the toner particles on the development sleeve scatter it is presumed as another cause of the above described phenomenon that the higher the process speed the shorter the time it takes for the toner particles on the development sleeve to pass through the contact area between the development blade and development sleeve and therefore the less likely the toner particles are to be charged thus in this embodiment as the cumulative amount a of a photoconductive drum usage reaches the threshold stored in the memory the following steps are taken in the listed order to switch the dc voltage as a part of the charge bias and the dc voltage as a part of.