A light-shelf is a highly efficient daylighting system about which a great deal of research has been performed. However, improvements to the daylighting performance of light-shelves have been limited, because light-shelves could introduce external natural light into the interior space through specular reflection, rather than into a specific region of the interior environment. In this study, to improve the performance of a light-shelf, a light-shelf using the principle of diffused reflection was proposed, and its validity was verified through performance verification. The conclusions of this study are as follows: (1) The optimal angles of a flat type light-shelf and a light-shelf using diffused reflection could vary depending on measurements taken in different seasons. (2) With the installation of the light-shelf using diffused reflection in the summer, the uniformity ratio of illuminance could be improved by 29.9%–34.3% with the external type and by 10.4%–13.7% with the internal type compared to the existing flat type light-shelf. (3) For the light-shelf using diffused reflection, the lighting energy could be reduced by 7.98%–13.3% in the case of the external type and by 1.8%–4.4% in the case of the internal type compared to the existing flat type light-shelf.