The present invention relates to a semi-cylindrical solar collecting apparatus for a solar boiler that is provided with a plurality of solar collecting plates having a semi-cylindrical shape, thereby greatly enhancing solar collecting efficiencies, and that connects a plurality of heat-exchanging pipes adapted to be heated by the plurality of solar collecting plates with an underground heat-exchanging pipe by means of auxiliary pipes, such that water is circulated well by means of a circulation pump, thereby completely solving the frozen and burst problems of the heat-exchanging pipes and circulation pipes.