The peripheral dose, defined as the dose outside therapeutic photom fields, Which is responsible for the functional damage of the critical organs, fetus, and radiation-nduced carcinogenesis, has been investigated for 60 Co gamma ray and 10 MV X ray. It was measured by silicon diode controlled by semiautomated water phantom without any shielding or with lead plate of HVL thickness put horizontally or vertically to shield stray radiations. Authors could obtain following results. 1. The peripheral dose was larger than 0.7% of central axis maximum dose even at 20cm distance from field margin. That is clinically significant, so it should be reduced. 2. Even for square fields of 10MV X ray, radial peripheral dose distribution did not coincide with transverse distribution, because of the position of collimator jaws. 3. Between surface and dm the peripheral dose distributions show a pattern of the dose distribution of electron beams and the maximum dose was approximately proportional to the length of a side of square filed. 4. The peripheral doses depended on radiation quality, field size, distance from field margin and depth in water. Distance from field margin was the most important factor. 5. Except for near surface, the peripheral dose from phantom was approximately equal to that from therapy unit. 6. To reduce the surface dose outside fields, therapist should shield stray radiations from therapy unit by lead plate of at least one HVL for 10 MV X-ay and by bolus equivalent to tissue of 0.5cm thickness for 60 Co. 7. To reduce the dose at depth deeper than dm, it is desirable to shield stray radiations from therapy unit by lead.