1. An image processing apparatus for synthesizing a plurality of image data, comprising: storage means for storing at least first image data and second image data each containing a plurality of pixels, and transmittance data set with respect to the pixels of at least said first image data; computing means for computing a distribution rate of color data of each of the pixels contained in said first and second image data based on said transmittance data set with respect to the pixels of said first image data, when said first image data is to be synthesized with said second image data; and synthesizing means for computing a color data value of each of a plurality of synthesized pixels obtained by synthesizing said first image data with said second image data based on said computed distribution rates of the respective color data of the respective pixels contained in said first and second image data, and for synthesizing said first image data with said second image data based on said computed color data values of said synthesized pixels; wherein said transmittance data is set to extra bit data exceeding a preselected color resolution among predetermined bit data constituting the color data of the respective pixels contained in said first image data.