PURPOSE: To obtain the subject compound without purification in excellent productivity, by inoculating Aspergillus niger B60 between tubes of a double capillary reactor, supplying O2 to the external part of the outer tube, firstly supplying a liquid medium, then a liquid medium deficient in NH4NO3 and culturing.

CONSTITUTION: Aspergillus niger B60 is inoculated between capillaries 2 (e.g. polypropylene capillaries) of a double capillary reactor 10 and external pipes 1 (e.g. silicone tubes), a liquid medium is supplied from a liquid medium injection hole 4a to the interiors of the capillaries 2, oxygen is fed from an oxygen feeding tool 3a to the external part of the external tubes 1 to culture Aspergillus niger 60B in high concentration. Then, a liquid medium deficient in NH4NO3 is fed to the interior of the capillaries 2 while supplying oxygen 3 to the external part of the external tubes 1 to continuously obtain citric acid from an outlet 4b for citric acid.