1. A color image forming apparatus forming a full color image by superimposing images of individual colors, comprising: a plurality of image forming devices including image bearing members configured to rotate or to move, development devices, and transfer devices, and configured to form images of individual colors on the image bearing members by illuminating the image bearing members with image light according to image data of individual colors to form latent images of individual colors on the image bearing members and to develop the latent images of individual colors on the image bearing members with toner by the development devices, respectively; a transfer member driven to rotate or to move, wherein the images of individual colors on the image bearing members of the plurality of image forming devices are sequentially transferred by the transfer devices of the plurality of image forming devices directly onto a recording sheet being conveyed by the transfer member while being superimposed on top of one another on the recording sheet, and thereby a full color image is formed on the recording sheet, or the images of individual colors on the image bearing members of the plurality of image forming devices are sequentially transferred by the transfer devices of the plurality of image forming devices once onto the transfer member while being superimposed on top of one another on the transfer member and then the images of individual colors superimposed on top of one another are transferred onto a recording sheet, and thereby a full color image is formed on the recording sheet; a detection device; a correction device configured to perform an operation of correcting deviations in position and errors in magnification ratio of images of individual colors that are superimposed on top of one another to form a full color image, wherein in the operation, a full color pattern image for correcting of a pattern of lines is formed on the transfer member by forming latent pattern images for correcting of individual colors on the image bearing members of the plurality of image forming devices, the latent pattern images for correcting of individual colors are developed on the image bearing members with toner into pattern images for correcting of individual colors by the development devices of the plurality image forming devices, the pattern images for correcting of individual colors are transferred by the transfer devices of the plurality of image forming devices while being superimposed one on top of another on the transfer member, and each line of the pattern of lines of the pattern image for correcting formed on the transfer member is detected with the detection device to determine a width of each line of the pattern of lines of the pattern image for correcting on the transfer member; and a correction possibility determination device configured to determine whether or not deviations in position and errors in magnification ratio of images of individual colors that are superimposed on top of one another to form a full color image can be correctly corrected, respectively, by: forming a full color pattern image for checking of a pattern of lines on the transfer member by forming latent pattern images for checking of individual colors on the image bearing members of the plurality of image forming devices, developing the latent pattern images for checking of individual colors on the image bearing members with toner into pattern images for checking of individual colors by the development devices of the plurality of image forming devices, transferring the pattern images for checking of individual colors on the image bearing members by the transfer devices of the plurality of image forming devices while being superimposed one on top of another on the transfer member, detecting each line of the pattern of lines of the pattern image for checking formed on the transfer member with the detection device to output a detection signal of each line of the pattern of lines of the pattern image for checking formed on the transfer member, and determining if a width of each line of the pattern of lines of the pattern image for checking formed on the transfer member is equal to or greater than a reference value based on the detection signal of each line of the pattern of lines of the pattern image for checking formed on the transfer member, wherein the correction possibility determination device determines that deviations in position and errors in magnification ratio of images of individual colors that are superimposed on top of one another to form a full color image can be correctly corrected, respectively, when the width of each line of the pattern of lines of the pattern image for checking formed on the transfer member has been determined to be equal to or greater than the reference value, and that deviations in position and errors in magnification ratio of images of individual colors that are superimposed on top of one another to form a full color image cannot be correctly corrected, respectively, when the width of any one line of the pattern of lines of the pattern image for checking formed on the transfer member has been determined to be not equal to or greater than the reference value, wherein the correcting deviations in position and errors in magnification ratio of images of individual colors that are superimposed on top of one another to form a full color image is performed by the correction device when the correction possibility determination device has determined that the width of each line of the pattern of lines of the pattern image for checking formed on the transfer member is equal to or greater than the reference value, and thereby that deviations in position and errors in magnification ratio of images of individual colors that are superimposed on top of one another to form a full color image can be correctly corrected, respectively.