1. An image forming apparatus comprising: at least one first image carrier configured to carry an electrostatic latent image thereon; an image writing unit configured to write the electrostatic latent image onto the at least one first image carrier, the electrostatic latent image including a test pattern; a second image carrier configured to move along a transfer position facing to the at least one first image carrier to obtain a subject image by transferring the electrostatic latent image carried on the at least one first carrier onto the second image carrier in a superimposing manner; an image forming unit provided in contact with the second image carrier and configured to transfer the subject image transferred on the second image carrier to a transfer material and convey the transfer material; a detector configured to detect the test pattern image; and a controller configured to correct an image forming condition of the subject image based on the detection result from the detector, wherein during a time period from the start of detecting the test pattern by the detector to the start of writing a subsequent subject image onto the at least one first image carrier by the image writing unit, the controller calculates an amount of at least one correction matter from the detection result from the detector, and reflects the calculated amount in the image forming condition of the subject image.