Reference field for mammography has been established using a molybdenum-anode X-ray tube and a molybdenum filter (Mo/Mo). The calibration coefficients of an ionization chamber obtained in the mammography reference field are compared with those in the conventional soft X-ray reference field which were produced using a tungsten-anode X-ray tube with aluminum filters (W/Al). The difference in the calibration coefficients between these reference fields was about 1 %. This development is expected to improve the precision of dosimetry for mammography.