Patent ID: 7444089

Claim:
An image forming apparatus that forms an image according to an electrophotographic system, comprising: a cylinder that freely rotates a fixed angle at a time around a rotation shaft horizontally arranged, the cylinder having a window in a side thereof; plural developing units that are arranged inside of the cylinder and rotate together with the cylinder and to which toner cartridges are detachable and attachable, each toner cartridge comprising: a window section of a translucent material that faces the window when the toner cartridge is arranged in each of the developing units; a reflection wall that faces the window section and is provided inside of the toner cartridge to form a fixed gap between the window section and the refection wall and the window section side of which reflects light; and a collection wall that is connected to the reflection wall and is provided inside of the toner cartridge so that one end of the collection wall is closer to an outer wall of the toner cartridge than the other end of the collection wall; and a toner level detecting system comprising: an optical sensor immovably provided outside of the cylinder, the optical sensor including a light emitting device that emits light to the reflection wall through the window and the window section; and a light receiving device configured to measure an intensity of reflected light from the reflection wall; a rotation signal outputting unit configured to output ON/OFF signals for rotation of the cylinder; a count clock configured to set a sampling clock of the light receiving device; a counter configured to count a toner flow-down time from the intensity of reflected light received by the light receiving device after receiving the OFF signal until the ON signal; a threshold setting unit that sets threshold values for judging toner levels; a register configured to extract the threshold values; a comparator that compares a count calculated by the counter and a threshold value read in the register to judge a toner level; and a toner level display unit configured to display a toner level calculated by the comparator, wherein the toner level detecting system detects toner levels of the toner cartridges arranged in the developing units by rotating the cylinder a predetermined angle to stop the cylinder and measuring flow-down times of toners, which flow down inside of the toner cartridges, using the optical sensor to detect a difference of intensities of reflected light caused by presence and absence of the toner in the gap.