1. An image forming apparatus, comprising: a plurality of image carriers to carry an image thereon; a plurality of drivers configured to drive each of the plurality of image carriers; a plurality of drive-force transmitting members configured to transmit a driving-force from the plurality of drivers to the plurality of image carriers; a developing unit, provided to each of the plurality of image carriers, configured to develop the image on each of the plurality of image carriers; a transfer member, being faced to the plurality of image carriers, configured to receive the developed image from each of the plurality of image carriers sequentially while endlessly moving in a given direction; an image detector configured to detect a detection image in the developed image formed on the transfer member in order to check a detection timing of the developed image; a sensor, provided to each of the plurality of image carriers, configured to detect a rotational speed of each of the plurality of image carriers and to determine a rotational angle of each of the plurality of image carriers; and a controller configured to conduct an image-to-image displacement control, a speed-deviation checking, and a phase adjustment control, the image-to-image displacement control including an image forming of the detection image on the transfer member, the detection image including an image transferred from all of the plurality of image carriers so that the transferred images from all of the plurality of image carriers are spaced apart on the transfer member, a detection of the detection image with the image detector, and an adjustment of image forming timing on each of the plurality of image carriers based on distances between the transferred images spaced apart on the transfer member; wherein the speed-deviation checking includes image forming of a speed-deviation checking image on the transfer member transferred from all of the plurality of image carriers, the speed-deviation checking image including the image transferred from all of the plurality of image carriers so that the transferred images from all of the plurality of image carriers are spaced apart on the transfer member, detecting the speed-deviation checking image with the image detector, and determining a speed-deviation of each of the plurality of image carriers per one revolution based on a result detected by the image detector and a result detected by the sensor, wherein the phase adjustment control includes a phase adjustment of each of the plurality of image carriers based on a result determined by the speed-deviation checking, and wherein the controller conducts the phase adjustment control and the image-to-image displacement control before conducting an image forming operation on each of the plurality of image carriers.