When conducting a color-shift-correcting mode using a registration mark
formed by the image forming section on an image carrier, the image
detecting section outputs the image detection signal representing the
line width of the registration mark and the control section calculates
the line width of the registration mark from the image detection signal.
The control section compares a line width of the registration mark with a
predetermined reference value of the line width to discriminate between
abnormality and normality. If normality, the control section continues
the color-shift-correcting mode and if abnormality the control section
conducts the image forming system restoration mode after stopping the
execution of the color-shift-correcting mode. Therefore, even in the case
where a white spot is generated for some reasons on the registration
mark, the color-shift-correcting mode can be conducted on the normal
registration mark with the help of the image forming system restoration
mode.