The microbial fuel cell (1) is provided with an electrolytic solution (2) containing an organic compound, a negative electrode (3) holding an anaerobic microorganism and contacting the electrolytic solution, and a positive electrode (4) provided with a water-repellent layer (41) and a gas diffusion layer (42) superposed on the water-repellent layer. In addition, when T1 represents the ratio of the negative electrode surface area to the gas diffusion layer surface area, and T2 represents the ratio of the positive electrode maximum current density at the potential of the positive electrode in the electrode system constituted by the electrolytic solution, the negative electrode and the positive electrode, to the negative electrode maximum current density at the potential of the negative electrode in the electrode system, T1 and T2 satisfy the relationship of formula (1): T2 1/2 <= T1 <= T2 2 (1)