We are interested in energy conversion mechanisms of biological nanomachines, particularly that of the bacterial flagellar motor. We would like to see the behavior of energy transduction system that drives the flagellar motor rotation. For this purpose we have been developing a system that combines an optics to measure the single motor rotation and an electrophysiological device to measure the proton current through a single motor. We expect that simultaneous measurement of the current and the rotation will give us crucial clues to the mechanism of the energy conversion.