The challenge of the present invention is to suppress a variation in brightness of an image and make a reference value of a black level converge at an appropriate value in a short time. A condition judgment circuit judges whether or not a frame of an amount of change in gains of a variable gain amplifier being equal to or greater than a threshold continues for a predefined frames or more. If a frame of an amount of change in the gains being equal to or greater than the threshold continues for the predefined frames, a black level value of the current frame is set for new black level reference. If not continues for the predetermined number, the previous black level reference value is retained in lieu of correcting the black level.