Coulometric analysis is effective in the analysis of an analyte in a liquid plug of a very small volume. However, in the conventional coulometric detection, it took time to collect analyte molecules dissolved in the solution. To solve this problem, we used a novel method to achieve significant enhancement of sensitivity by using a one-electrode system. As a model analyte, hydrogen peroxide was oxidized in a flow channel and silver was deposited simultaneously on another electrode in another flow channel. Coulometric detection of the deposited silver showed a step change of generated charge. As a result, measurement time was reduced significantly (less than 2 s), although conventional methods took more than 60 s to achieve the same sensitivity. This shorter measurement time also suppressed the influence of background noise, resulting in significant lowering of the detection limit.