Abstract: A process for producing formic acid by hydrothermal oxidation reaction with glycerol and their equipment, wherein glycerol and oxidant are subjected to a hydrothermal oxidation reaction, air, oxygen or H2O2 as oxidant, at a temperature in the range of 150 to 450° C. and under pressure equal to or more than the saturated vapor pressure at the temperature. The glycerol produced from plant fats, animal fats or the like or pure product synthesized chemically or a discharge containing glycerol generated at the production of diesel fuel oil from fats, in which the fats are subjected to a transesterification with alcohol in the presence of an alkali catalyst in order to obtain fatty acid ester is preferably used as a starting material.