Power generation efficiency of a microbial power generation device is improved by a simple and inexpensive means. Two plate-shaped cation-exchange membranes 31 are disposed parallel to each other in a tank body 30, whereby a negative electrode chamber 32 is formed between the cation-exchange membranes 31. Two positive electrode chambers 33 are each formed so as to be separated from the negative electrode chamber 32 by the corresponding cation-exchange membrane 31. An oxygen-containing gas is passed through the positive electrode chamber 33, a negative electrode solution L is supplied to the negative electrode chamber, and preferably the negative electrode solution is circulated. An acid gas (carbon dioxide gas) is introduced into the oxygen-containing gas to be supplied to the positive electrode chamber 33. Movement of Na+ and K+ ions is promoted by the pH neutralization effect produced by the acid gas, and thereby power generation efficiency can be improved.