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.