To prevent inhibition of methane fermentation by free ammonia.

After pH, total ammonia, total VFA (total volatile organic acid), etc., are measured by a measuring process 1, free ammonia, nonelectrolytic VFA, etc., are calculated based on the measurement of the measuring process 1 by a calculating process 2 and the volume load of organic materials is limited by a load range limiting process 3. Then, the relation of free ammonia and the amount of methane formed per fed organic materials in a load range limited by the range limiting process 3 is subjected to correlation analysis by a correlation analysis process 4. After the correlation analysis by the process 4, the presence of the initial inhibition by free ammonia and the degree of inhibition are judged by a judging process 5 and the result is outputted.