NOVELTY - A methane gas concentration apparatus comprises compressor (11) which pressurizes or sends source gas containing carbon dioxide gas and methane gas, methane gas concentration system (10) which separates source gas supplied from compressor from methane gas by gas separation membrane (12) which permeates carbon dioxide gas, methane gas purification system (20) which separates enriched gas of methane gas concentration system from methane gas by gas separation membrane (21, 22), and methane gas recovery system (30).    USE - A methane gas concentration apparatus (claimed).    ADVANTAGE - High concentration of methane gas can be stably produced.    DETAILED DESCRIPTION - A methane gas concentration apparatus comprises compressor (11) which pressurizes or sends source gas containing carbon dioxide gas and methane gas, methane gas concentration system (10) which separates source gas supplied from compressor from methane gas by gas separation membrane (12) which permeates carbon dioxide gas, methane gas purification system (20) which separates enriched gas of methane gas concentration system from methane gas by gas separation membrane (21, 22), methane gas recovery system (30) which separates permeated gas of methane gas concentration system from methane gas by gas separation membrane (31), return flow path (28) which returns permeated gas of methane gas purification system and enriched gas of methane gas recovery system to compressor, 1st detection apparatus (41, 44) which detects flow volume and concentration of methane gas or carbon dioxide gas concentration of source gas supplied to methane gas concentration system, 2nd detection apparatus (42, 45) which detects flow volume and concentration of methane gas or carbon dioxide gas concentration of enriched gas of methane gas purification system, and 3rd detection apparatus (43, 46) which detects flow volume and concentration of methane gas or carbon dioxide gas concentration of permeated gas of methane gas recovery system. Methane gas purification system and/or methane gas recovery system can adjust area of gas separation membrane.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the methane gas concentration apparatus. (Drawing includes non-English language text).    Methane gas concentration system (10)    Compressor (11)    Gas separation membrane (12)    Methane gas purification system (20)    Gas separation membrane (21, 22)    Return flow path (28)    Methane gas recovery system (30)    Gas separation membrane (31)    1st detection apparatus (41, 44)    2nd detection apparatus (42, 45)    3rd detection apparatus (43, 46)