The photocatalytic reduction of CO2 was investigated using TiO2 powders in supercritical fluid CO2. These were irradiated in a stainless steel vessel at 9.0 MPa and 35 degrees C, After reducing the CO2 pressure to the ordinary state, pure water was added to the vessel while avoiding air contamination. No gaseous reduction products were observed. Formic acid was obtained only in aqueous solution. The optimal irradiation time for the production of formic acid was 5 h, Addition of acidic solutions rather than pure water was preferable for formic acid formation. Formic acid seems to be produced through; the protonation of reaction intermediates on TiO2 powders in solutions. The CO2-reduction system described here may be of practical value for efficient CO2-conversion and fixation, storage of solar energy, and production of raw materials for the photochemical industry.