PURPOSE: To provide a apparatus for producing formic acid from CO and methanol with decreased steam consumption, by using an assistant reboiler of methyl formate distrillation column heated with the material evaporated from a formic acid distillation column, and a means for the heat-exchange between the reaction liquid and the condensate of a low-boiling distillation column.

CONSTITUTION: A reaction liquid containing CO, methanol and a catalyst is circulated through the assistant cooler 40 cooled by the condensate of the low-boiling distillation column condenser 11, the cooler 2 and the reactor 1, and is made to react while remov ing the heat of reaction to produce methyl formate. The reaction product is transferred to the methyl formate distillation column 4, heated and distilled by the heat of the assistant reboiler 41 heated with the evaporation product of the reboiler 5 and the formic acid distillation column 13 heated with steam. Methyl formate distilled from the column top is condensed and hydrolyzed (8). The bottom liquid is transferred to the flush drum 42 to evaporate and remove the low-boiling components composed mainly of methyl formate, and the residual liquid is transferred through the low-boiling distillation column 9 to the formic acid distillation column 13. The evaporation product of the distillation column 13 is condensed by the heat-exchange with the assistant reboiler to recover the objective formic acid.