An image forming apparatus includes an image carrier on which an
electrostatic latent image is formed, a rotary developing device having N
developer carriers on an rotation orbit and moving the N developer
carriers to a development position opposite to the image carrier, and a
toner concentration sensor that measures toner concentrations of the
developers. A measurement position of the toner concentration sensor is
set on a second virtual straight line having a first angle toward a first
virtual straight line that connects a rotation center of the rotary
developing device and the development position. The N developer carriers
are placed in turn in an order at the same angular interval as the first
angle, and an angular interval between the N-th and first developer
carriers is set to a second angle greater than the first angle.