To provide an apparatus for monitoring a concentration of a volatile organic acid in a fermentation liquid which can accurately monitor the concentration of the volatile organic acid in the fermentation liquid so that fermentation may be stabilized and an yield of biogas may be increased.

The apparatus for monitoring the concentration of the volatile organic acid in the fermentation liquid includes: a carbon acid trapping means 2 for trapping a carbon acid contained in the fermentation liquid sampled from a fermenter, as an extremely insoluble salt in a solid phase by adding a basic carbon acid trapping agent; a solid-liquid separation means 3 for removing the extremely insoluble salt by solid-liquid separation; a volatile organic acid diffusing means 4 for diffusing the volatile organic acid into a gas phase by adding an non-volatile acid to a liquid phase after the solid-liquid separation; and a quantifying means 5 for quantifying the concentration of the volatile organic acid diffused into the gas phase.