This invention provides an image processing device and an image processing method capable of performing image processing of higher accuracy using the synthetic image data with enlarged dynamic range. An image processing method including the steps of acquiring a plurality of input image data with pixel information defined with a first tone range by performing imaging over plural times at different exposure conditions using an imaging unit, generating synthetic image data with pixel information defined with a second tone range wider than the first tone range by synthesizing the plurality of input image data, generating processed image data containing pixel information defined with the second tone range by performing image processing on the synthetic image data for facilitating detection or measurement related to an imaged target; and converting the processed image data to output image data containing pixel information defined with a third tone range narrower than the second tone range is disclosed.