A positive electrode, a negative electrode, and an electrolyte intervening between the positive electrode and the negative electrode are employed, and a molecule capable of being excited due to absorption of light and electrochemically oxidizing carbohydrate is provided at at least either the negative electrode or the electrolyte, with production of electromotive force occurring between the positive electrode and the negative electrode as a result of supply of carbohydrate while the molecule is irradiated with light and oxidization of carbohydrate by the molecule at the negative electrode. This method makes it possible for the chemical energy which carbohydrates possess to be directly utilized as electrical energy.