1. A medical image processing apparatus comprising: an image storage unit configured to store a first medical image and a second medical image having pixels respectively originating from X-rays of a first energy and a second energy; a conversion unit connected to said image storage unit for converting a pixel value of the first medical image and the second medical image into a mass attenuation coefficient for each of the pixels to generate a first attenuation map and a second attenuation map; a quotient calculation unit connected to said conversion unit for generating a quotient based upon the first attenuation map and the second attenuation map and storing the quotient in a division map; an estimation unit connected to said quotient calculation unit for estimating an abundance ratio of one of a first substance and a second substance to the other for each of the pixels based on the quotient in the division map, a first attenuation coefficient ratio of the first substance which correspond to the first energy and the second energy, and a second attenuation coefficient ratio of the second substance which correspond to the first energy and the second energy, said estimation unit comparing the quotient with at least one of the first attenuation coefficient ratio (As) and the second attenuation ratio (Bs) to estimate the abundance ratio; a combination ratio determination unit configured to determine combination ratios of pixel values between the first medical image and the second medical image for each of the pixels based on the abundance ratios of the first substance and the second substance and normalized attenuation coefficients of the first substance and the second substance which are associated with a target energy; and a combined image generation unit configured to generate a combined image associated with the target energy from the first medical image and the second medical image by using the determined combination ratios.