Postoperative radiotherapy of breast cancer makes it possible to reduce loco-regional recurrence of breast cancer. The treatment technique, which can reduce the 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 $^{60}Co;gamma-ray$ on internal mammary region and 7MeV electron on chest wall. Exposure time of $^{60}Co;gamma$ and monitor unit of 9MeV were selected so that dose of $^{60}Co$ at 4cm depth was the same as that of 7Mev electron at $80\%$ dose depth. The position and direction of electron beam were changed for $^{60}Co$ beam: $0^{circ},;5^{circ}$ for 0cm seperation; $0^{circ},;5^{circ},;10^{circ}$ for 0.5cm seperation; $5^{circ},;10^{circ},;15^{circ}$ for 1cm seperation. The results are as followings. 1. When the seperation of two fields was increased, dose on the axis of $^{60}Co$ 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 $0~0.5cm$ and the direction of electron beam was parallel to $^{60}Co$ beam.