To increase the power generation efficiency of a microbial power generator by using an easy and inexpensive unit. Two plate-like cation-exchange membranes 31 are disposed in parallel in a tank 30. This arrangement allows an anode chamber 32 to be formed between the cation-exchange membranes 31. Two cathode chambers 33 are separated from the anode chamber 32 by using the respective ion-permeable nonconductive membranes 31. An oxygen-containing gas is made to pass through the cathode chamber 33. An anode solution L is supplied to the anode chamber, and, preferably, the anode solution is made to circulate. A biologically treated exhaust gas is used as the oxygen-containing gas to be supplied to the cathode chamber 33. Carbon dioxide in the biologically treated exhaust gas can promote transport of Na+ and K+ ions, and water vapor can increase the ion permeability, thereby increasing the power generation efficiency.