This invention provides an image processing device and an image processing method capable of generating synthetic image data containing effective image information without being influenced by the variation of the photographing environment. A correcting portion updates (shifts) a “brightness” upper and lower limit range according to a deviation ΔE received from a deviation calculating portion. The correcting portion accepts an initial value Emax0 of the “brightness” upper limit and an initial value Emin0 of the lower limit in advance, so that the “brightness” upper limit and lower limit after update is (Emax0+ΔE) and (Emin0+ΔE), respectively. The “brightness” upper and lower limit range after the update calculated by the correcting portion is provided to a tone mapping unit.