Postoperative radiotherapy of breast cancer makes it possible to reduce loco-regional recurrence of breast cancer. The treatment technique, which can low-dose region at the junction and lung, is required. To produce proper dose distribution of internal mammary chain and chest wall, authors tried to find the method to expose 60Co gamma-ray on internal mammary region and 7 MeV electron on chest wall. Exposure time of 60Co and monitor unit of 9 MeV were selected so that dose of 60Co at 4 cm depth was the same as that of 7 Mev elevtron at 80% dose depth. The position and direction of electron beam were changed for 60Co beam: 0 degrees, 5 degrees for 0 cm seperation; 0 degrees, 5 degrees, 10 degrees for 0.5 cm seperation; 5 degrees, 10 degrees, 15 degrees for 1 cm seperation. The results are as followings. 1. When the seperation of two fields was increased, dose on the axis of 60Co beam was increased and dose at the junction region decreased while the volume of lung to be exposed to high dose and hot spot size were irregularly changed. 2. The dose distribution in the target volume of internal mammary and chest wall was most ideal when the seperation of two fields was 0 - 0.5 cm and the direction of electron beam was parallel to 60Co beam.