To provide an operating method of a VOC gas treating apparatus for performing a stable and economical operation while replenishing the absolute minimum amount of nutrients to microorganisms.

In the operating method of the VOC gas treating apparatus for collecting an exhaust gas containing VOC, guiding it to a biological treatment tank 1 filled with a carrier to which the microorganisms are attached and biodegrading it, a storage water tank 13 for storing circulation water c containing the nutrient d is provided on the bottom part of the biological treatment tank 1; a concentration measuring instrument 5 for measuring the ammonium ion concentration and/or phosphate ion concentration of the circulation water c inside the storage water tank 13 and a nutrient injecting apparatus for injecting the nutrient d containing ammonium ions and/or phosphate ions to the storage water tank 13 are provided, and the nutrient injecting apparatus is operated to inject the nutrient d to the storage water tank 13 so that the value of the concentration measuring instrument 5 does not become less than a prescribed value.