An image forming apparatus for starting image formation on the basis of a print command, comprising: first and second photosensitive members ( 304, 303; 302, 301); first and second light sources ( LD4, LD3; LD2, LD1) each for emitting light; a rotatable polygonal mirror including a plurality of reflecting facets, wherein said rotatable polygonal mirror deflects the light, emitted from said first light source, toward said first photosensitive member and deflects the light, emitted from said second light source, toward said second photosensitive member; first signal output means for outputting a first signal by receiving, in a predetermined position, the light emitted from said first light source and deflected by said rotatable polygonal mirror; and second signal output means ( 402, 403) for outputting a second signal on the basis of the first signal, wherein said first light source ( LD4, LD3) emits the light at timing on the basis of the first signal and said second light source ( LD2, LD1) emits the light at timing on the basis of the second signal so that latent images are formed on said first and second photosensitive members ( 304, 303; 302, 301), respectively, and then are developed with developers to form developer images on said first and second photosensitive members ( 304, 303; 302, 301), respectively, and then an image is formed by superposing the developer images, and wherein the light emitted from said first light source ( LD4, LD3) and the light emitted from said second light source ( LD2, LD1) at the same timing are incident on different reflecting facets of said rotational polygonal mirror to illuminate said first and second photosensitive members ( 304, 303; 302, 301) therewith, respectively; said second signal output means ( 402, 403) derives a correction value, for correcting the first signal, with respect to every reflecting facet of said rotational polygonal mirror, and then generates and outputs the second signal with respect to every reflecting facet of said rotational polygonal mirror on the basis of the correction value and the first signal; in a period in which a step of deriving the correction value by said second signal output means ( 402, 403) is executed, said first light source ( LD4, LD3) starts emission on the basis of the first signal to form the latent image on said first photosensitive member ( 304, 303); and before output of the second signal is started after said image forming apparatus receives the print command, said first light source ( LD4, LD3) starts light emission on the basis of the first signal to form the latent image on said first photosensitive member ( 304, 303). first and second photosensitive members ( 304, 303; 302, 301); first and second light sources ( LD4, LD3; LD2, LD1) each for emitting light; a rotatable polygonal mirror including a plurality of reflecting facets, wherein said rotatable polygonal mirror deflects the light, emitted from said first light source, toward said first photosensitive member and deflects the light, emitted from said second light source, toward said second photosensitive member; first signal output means for outputting a first signal by receiving, in a predetermined position, the light emitted from said first light source and deflected by said rotatable polygonal mirror; and second signal output means ( 402, 403) for outputting a second signal on the basis of the first signal, wherein said first light source ( LD4, LD3) emits the light at timing on the basis of the first signal and said second light source ( LD2, LD1) emits the light at timing on the basis of the second signal so that latent images are formed on said first and second photosensitive members ( 304, 303; 302, 301), respectively, and then are developed with developers to form developer images on said first and second photosensitive members ( 304, 303; 302, 301), respectively, and then an image is formed by superposing the developer images, and wherein the light emitted from said first light source ( LD4, LD3) and the light emitted from said second light source ( LD2, LD1) at the same timing are incident on different reflecting facets of said rotational polygonal mirror to illuminate said first and second photosensitive members ( 304, 303; 302, 301) therewith, respectively; said second signal output means ( 402, 403) derives a correction value, for correcting the first signal, with respect to every reflecting facet of said rotational polygonal mirror, and then generates and outputs the second signal with respect to every reflecting facet of said rotational polygonal mirror on the basis of the correction value and the first signal; in a period in which a step of deriving the correction value by said second signal output means ( 402, 403) is executed, said first light source ( LD4, LD3) starts emission on the basis of the first signal to form the latent image on said first photosensitive member ( 304, 303); and before output of the second signal is started after said image forming apparatus receives the print command, said first light source ( LD4, LD3) starts light emission on the basis of the first signal to form the latent image on said first photosensitive member ( 304, 303).