A signal processing apparatus which is capable of performing a smear amount correction processing suitably according to change in the smear amount to suppress overcorrection of the smear amount. An output value of an image sensor and a smear amount on each of vertical lines of the image sensor are stored, respectively. An output signal value of an optical black portion on the image sensor stored in the first memory is compared with the smear amount stored in the second memory to determine a cyclic coefficient based on the comparison result. The smear amount is calculated based on the output signal value of the optical black portion on the image sensor stored in the first memory, the smear amount stored in the second memory, and the cyclic coefficient determined by the smear detection unit. A correction coefficient is calculated based on the smear amount determined by the smear amount calculating unit. The output signal value on an effective pixel portion of the image sensor stored in the first memory is subjected to a correction processing using the correction coefficient determined by the correction coefficient calculating unit.