The kinetic properties of formate reduction to formaldehyde with formaldehyde dehydrogenase (FldDH) using single-electron reduced methylviologen (MV˙) as a co-enzyme were clarified. By using natural co-enzyme NADH, actually, no formaldehyde production due to formate reduction with FldDH was observed. In contrast, we discovered that MV˙ activates the FldDH activity for formate reduction to formaldehyde for the first time.