A processing apparatus obtains pixel signals of a plurality of images by shooting a subject with different sensitivities by a sensor having a plurality of pixels; obtains a conversion ratio for converting the pixel signals of the plurality of images into pixel signals with a same sensitivity and a synthesis ratio for synthesizing the pixel signals of the plurality of images with the same sensitivity for each pixel; and performs dynamic range expansion synthesis for each pixel using the pixel signals of the plurality of images using the conversion ratio and the synthesis ratio. The synthesis ratio is obtained for each divided region obtained by dividing the plurality of pixels.