Control system for a methane fermentation vessel in which the activity of a methanogen is determined by measuring the fluorescence intensity of the fluorescent substance F420 to control the feed of the raw materials.    Also claimed are a method for the automatic control system for a methane fermentation vessel in which the fermentation liquor is sampled from a vessel and diluted or further alkali-treated or heated and then the fluorescence intensity of the fluorescent substance F420 is measured and the data is treated by a data processing equipment. The data are analyzed to control the flow controller and to control the feed amount and/or the feed time of the raw materials. Automatic control equipment consists of vessel for performing methane fermentation, a line for sampling the fermentation liquor from the methane fermentation vessel, a pretreating equipment for diluting or further alkali-treating or heating the fermentation liquor, a spectrophotometer for measuring the fluorescence intensity of the fluorescent substance F420, data processing equipment for analysing the fluorescence intensity data and a flow controller controlled by a control line based on the analytical date, and in which the activity of a methanogen is determined by measuring the fluorescence intensity of the fluorescent substance F420 and the feed of the raw materials is controlled according to it to control the methane fermentation.    USE/ADVANTAGE -   Methane fermentation is used for organic waste treatment and waste water treatment. The system can determine the activity of a methanogen easily.    In an example, waste material contg. 130000 mg/l T-COD, 50000 mg/l S-COD, 80000 mg/l T-BOD, 45000 mg/l S-BOD, 50000 SS and 50000 mg/l VSS was used as the raw material. A 3 litre methane fermentation vessel was used and operated at 55 deg.C. The pretreatment was carried out by sampling the fermentation liquor from the sampling line, diluting it 10 times, and adding NaOH to 1 N, standing it at 4 deg.C for 30 min., and filtering it through a 0.45 micron membrane and feeding it to a fluorescence spectrophotometer. A high temp. digested sludge was fed to the vessel and digested until no gas formed. Then, the equipment was started up while controlling the garbage amount. The start-up was completed in 40 days.