1. An image capturing system for medical use, comprising: a detecting unit for detecting an X-ray dose by absorbing X-rays passed through a subject in image capturing for medical use and outputting absorbed X-ray dose information; an obtaining unit for obtaining image information of said subject from said absorbed X-ray dose information; an image processing unit for calculating average detection energy in each first predetermined region in said image information, wherein said image processing unit estimates a number of photons of X-rays absorbed by said detecting unit using said image information to calculate said average detection energy in each said first predetermined region in said image information, and wherein said image information includes a plurality of pieces of image information obtained by performing image capturing for medical use using X-rays in energy regions, and said image processing unit estimates the number of photons of X-rays by using said plurality of pieces of image information and applied X-ray dose information as information of dose of X-rays outputted from an X-ray generator; and an output unit for outputting said average detection energy.