The shading correction is performed with respect to a signal "A" indicative of the image density read by an optical read device in accordance with a reference signal "B". Both the signals "A" and "B" consist of n bits. An inputted signal having an initial value "A" is doubled by a multiplier when A=B and B.noteq.0. A subtracter subtracts the reference signal "B" from a signal "X" outputted from the multiplier and discriminates whether or not a signal "X-B" is negative. An S/P conversion register doubles an integrated value "S" having an initial value "0" and changes the integrated value from "S" to "2S" when the signal "X-B" is negative. In contrast, when the signal "X-B" is not negative the S/P conversion register adds "1" to "2S" and changes the integrated value from "S" to "2S+1". The signal "X" outputted from the multiplier is again inputted into the multiplier when the signal "X-B" is negative. Alternatively, the multiplier receives the signal "X-B" outputted from the subtracter when the signal "X-B" is not negative. The, above procedure is repeated n times. The last integrated value outputted from the S/P conversion register is equal to or approximately equal to a value obtained by an expression "S=A(2.sup.n -1)/B", and therefore, the former can be regarded as a shading correction signal.