Phase contrast imaging has been studied intensively using X-rays from synchrotron radiation and microfocus x-ray tubes. We have created a new method of analyzing edge effect in phase contrast imaging with practical x-ray tubes for medical use. Based upon this theory, we have developed a full-field digital phase contrast mammography system in order to provide such high sharp images as conventional screen-film mammography. We would like to describe our basic concept for application of phase contrast technology to digital mammography, especially on magnification ratio in digital mammography.