1. A method of correcting an image signal produced by introducing light which bears image information carried by a subject into tricolor separating optical systems, reading the light one line at a time in a main scanning direction with linear image sensors for the three colors R, G, B, integral with the tricolor separating optical systems, moving the subject in an auxiliary scanning direction relatively to the tricolor separating optical systems for thereby two-dimensionally reading the image information carried by the subject, and alternately reading odd- and even-numbered pixel signals outputted from the linear image sensors 1 to produce the image signal which comprises a series of pixel signals, said method comprising the steps of: correcting an offset of each of the pixel signals of an image signal of a present line by supplying a transfer clock signal to determine an empty-transfer level after the pixel images of each line are read and calculating a corrective quantity for each of the pixel signals with a gradient average of an empty-transfer level of a preceding line and an empty-transfer level of the present line; correcting sensitivity variations of the pixels of the linear image sensors based on image signals produced when a white or transparent subject is read directly and through a filter having a predetermined density; correcting an auxiliary scanning curvature and odd- and even-numbered pixel level differences of the image signal by reading a subject carrying a linear image extending in the main scanning direction for successive lines with the image sensors, thereafter determining and correcting pixel shifts in the auxiliary scanning direction of two of the linear image sensors with respect to the remaining one of the linear image sensor used as a reference, and selectively outputting pixel signals which are to be corrected or pixel signals which have been corrected based on averages of and differences between the levels of the pixel signals which are to be corrected and the pixel signals which have been corrected for thereby correcting variations in the levels of the odd- and even-numbered pixel signals; correcting aberrations and magnifications in the main scanning direction by reading a reference chart having marks spaced at equal intervals in the main scanning direction with the linear image sensors and thereafter calculating electric magnification correction coefficients with respect to the respective pixels such that reproduced images of the marks based on an image signal representative of the read marks will be spaced at equal intervals; and converting a luminance-level image signal produced by the linear image sensors into a density-level image signal with a luminance-to-density conversion table.