NOVELTY - Biogas apparatus comprises a low concentration wastewater treatment tank, first water level sensor, high concentration wastewater treatment tank, second water level sensor, low concentration wastewater treatment tank and souring tank connected with the high concentration wastewater treatment tank, first pump, second pump, methane fermentation tank, purifier, gas engine generator, and control apparatus.    USE - The apparatus is useful for producing biogas.    ADVANTAGE - The apparatus has excellent fermentation efficiency, and controls mixing ratio of high concentration wastewater having high BOD with low concentration wastewater, and BOD and viscosity of wastewater.    DETAILED DESCRIPTION - Biogas apparatus (1) comprises a low concentration wastewater treatment tank (5) containing wastewater containing organic substance and retains low concentration wastewater having low biological oxygen demand (BOD), first water level sensor which measures water level of the low concentration wastewater treatment tank, high concentration wastewater treatment tank (6) which retains high concentration wastewater, whose BOD is higher than the low concentration wastewater, second water level sensor which measures water level of the high concentration wastewater treatment tank, low concentration wastewater treatment tank and souring tank (7) connected with the high concentration wastewater treatment tank, first pump provided between the low concentration wastewater treatment tank and the souring tank, second pump provided between the high concentration wastewater treatment tank and the souring tank, methane fermentation tank (11) for generating biogas connected with the souring tank, purifier connected with the methane fermentation tank, gas engine generator which burns the biogas and air and drives the gas engine and generator, and a control apparatus connected with the first water level sensor, second water level sensor, first pump and second pump. When the water level of the low concentration wastewater treatment tank is more than the first drive permission water level, the first pump is driven, and when the water level of the high concentration wastewater treatment tank is more than the second drive permission water level, the second pulp is driven.    DESCRIPTION OF DRAWING(S) - The drawing shows block diagram illustrating the biogas production apparatus.    Biogas apparatus (1)    Low concentration wastewater treatment tank (5)    High concentration wastewater treatment tank (6)    Souring tank (7)    Methane fermentation tank (11)