The power generation efficiency of a microbial power generator is increased by using an easy and inexpensive unit. Two plate-like cation-exchange membranes are disposed in parallel in a tank. This arrangement allows an anode chamber to be formed between the cation-exchange membranes. Two cathode chambers are separated from the anode chamber by using the respective ion-permeable nonconductive membranes. An oxygen-containing gas is made to pass through the cathode chamber. An anode solution 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. 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.