PURPOSE: To provide a method for producing hydrogen with a microorganism by which a high hydrogen yield closer to a theoretically obtained amount is obtained without producing lactic or formic acid.

CONSTITUTION: An organic substrate previously regulated so as to make lactic acid or a mixture thereof with formic acid present in an amount corresponding to 10-30%, expressed in terms of carbon and based on the fed organic substrate is used as the organic substrate in a method for fermenting an organic substrate and producing hydrogen. The organic substrate is preferably used by regulating the pH to neutral by adding an alkaline agent thereto.