1. A mammary gland content rate estimating apparatus, comprising: a breast image acquiring device which acquires a breast image obtained by radiographing a breast by a mammography imaging apparatus; an adipose image estimating device which estimates an adipose image from the acquired breast image based on an assumption that an entire breast is composed of only adipose tissues; a device which acquires a pixel value of a directly irradiated region from the acquired breast image; and a mammary gland content rate calculating device which calculates a mammary gland content rate for each of pixels in the breast image based on the acquired breast image, the estimated adipose image and the acquired pixel value of the directly irradiated region.