To remove volatile organic compounds efficiently by supplying gas containing volatile organic compounds to a pipeline for circulating liquid in a biological reaction tank which receives biological sludge which assimilates volatile organic compounds to dissolve the gas in the circulating liquid.

Liquid 2 containing biological sludge is placed in a biological reaction tank 1, the lower and upper parts of the tank 1 are connected by a circulating pipeline 3, and a pump 4 is installed in the pipeline 3. In the treatment of gas containing volatile organic compounds(VOC), the gas is supplied from a gas supply pipe 6 into the pipeline 3, the gas supplied is mixed with the liquid 2 while passing through the pipeline 3, and VOC and also oxygen in the gas are dissolved into the liquid 2. The liquid 2 is circulated into the tank 1 to contact the biological sludge so that VOC are taken in by the sludge to be assimilated.