Abstract: [PROBLEMS] To provide a microbial fuel cell whose parts can be replaced without lowering the energy recovery efficiency and a membrane cassette for microbial fuel cells. [MEANS FOR SOLVING PROBLEMS] A negative electrode (10) supporting anaerobic microorganisms (11) is immersed in an organic substrate (S). A positive electrode (15) sealed together with an electrolyte (D) in a closed hollow cassette (20) having an outer shell (25) at least a part of which is formed of an ion-permeable membrane (21), an inlet (22), and an outlet (23) or connected to the inner side of an ion-permeable membrane (21) is inserted into the organic substrate (S). While oxygen (O) is supplied into the cassette (20) through the inlet (22) and the outlet (23), electricity is taken out through a circuit (18) electrically interconnecting the negative and positive electrodes (10, 15).