To provide an image forming device avoiding erroneous detection in reading of each kind of adjustment patterns by a sensor and dispersion of correction even when an endless belt has a tendency to curl.

An image forming device comprises: an image carrier; a rotatable endless belt wrapped around at least two rollers; transferring means for transferring a toner image formed on the image carrier on a surface of the endless belt; and detecting means for detecting a curled portion of the endless belt formed by rollers, positional shift adjustment patterns and/or density adjustment patterns formed on the surface of the endless belt. In the image forming device, detecting operation for detecting the curled portion is performed by the detecting means by rotating the endless belt at least one rotation. When the curled portion is detected, the positional shift adjustment patterns and/or the density adjustment patterns are formed so as to avoid overlapping with the curled portion. When the endless belt is stopped from rotating, the endless belt is stopped so as to overlap the curled portion with positions of the rollers which have formed the curled portion.