An image forming apparatus includes an image carrier on which an electrostatic latent image is formed, a rotary developing device having N developer carriers, where N defines a total number of developer carriers, the N developer carriers carrying developers to develop the electrostatic latent image, and the rotary developing device moves 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 carried in the developer carriers. A measurement position of the toner concentration sensor is set on a second virtual straight line with the second virtual straight line having a first angle or an angle that is double the first angle toward a first virtual straight line on a rotation orbit of the rotary device, and the first virtual straight line connects a rotation center of the rotary developing device and the development position. The N developer carriers are placed in turn in an order starting from a first developer carrier to an N-th developer carrier at the same angular interval as the first angle on the rotation orbit of the rotary developing device, and an angular interval between the N-th developer carrier and the first developer carrier is set to a second angle greater than the first angle.