1. A medical image processing apparatus comprising: an image storage unit configured to store a first medical image and a second medical image respectively originating from X-rays of a first energy and a second energy; an estimation unit configured to estimate an abundance ratio of one of a first substance and a second substance to the other for each pixel based on attenuation coefficients of the first substance which correspond to the first energy and the second energy, attenuation coefficients of the second substance which correspond to the first energy and the second energy, and pixel values of the first medical image and the second medical image; 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 pixel based on the abundance ratios of the first substance and the second substance and the 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.