(57) An image forming apparatus comprises a photoreceptor (1); plural developer units
(2, 3) for giving toner alternatively to the photoreceptor (1) to develop a latent
image formed on the photoreceptor (1); toner density sensors (35, 37) for detecting
the toner density of the developer units (2, 3); toner supplier for supplying toner
to the developer units (2, 3) according to the output of the toner density sensor
(35, 37); detector (35, 47) for detecting whether said developer units (2, 3) are
attached with respect to the periphery of the photoreceptor (1); and controller (40)
for so controlling the image forming operation as to be performed only when a predetermined
plural number of developer units (2, 3) are attached.
According to the image forming apparatus, it is possible to correct the output of
the toner density sensor (35).
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