A solar-heat collecting apparatus of the direct conversion type wherein a black working fluid directly absorbs solar-heat. A plurality of solar-heat collecting tube units are arranged in parallel with each other within a box-like casing. Each tube unit comprises a transparent outer tube and an inner tube mounted within the outer tube with a bottom face of the inner tube being in contact with an inner face of the outer tube. Both ends of each inner tube are closed, forming a fluid passage between each inner and outer tube pair and each end of the respective outer tubes is connected with one of two headers.