An estimation selecting unit is provided to select an estimating method based on a predetermined value. When variations due to a statistical error of each pixel concerned are enlarged by a greater extent than variations of other pixels, and hence a possibility of becoming conspicuous on the image, then the direct ray transmittance at the pixel concerned is considered less than the predetermined value, and estimated direct ray intensity is obtained for the pixel concerned by interpolating calculation of estimated direct ray intensities at pixels surrounding the pixel concerned. Therefore, even when variations due to a statistical error of the pixel concerned are enlarged by a greater extent than variations of other pixels, and hence a possibility of becoming conspicuous on the image, estimated direct ray intensity is obtained for the pixel concerned by interpolating calculation of estimated direct ray intensities at pixels surrounding the pixel concerned, whereby the error can be inhibited without the pixel value of the pixel concerned being conspicuous relative to the surrounding pixels on the image.