An object of the present invention is to provide an image data processing circuit and an image data processing method capable of determining a reference signal level corresponding to black in an image with high precision by suppressing occurrence of line dependency and the like in a dark current component signal included in image data output from a solid state image pickup device. A reference dark current component data holding unit selects a reference line from a solid state image pickup device and holds a dark current component of the reference line as a head line average value. To a subtraction circuit, dark current component data and effective pixel data is sequentially input on the line unit basis. A differential circuit obtains, as a detection value, a change amount with respect to the head line average value, of the dark current component data included in a preceding line. The subtraction circuit subtracts the detection value from the dark current component data and the effective pixel data entered.