We report a micro-scale microbial fuel cell (MFC), having an ultramicroelectrode (UME) as its anode, aiming for a miniaturized energy converter of carbon-neutral renewable energy. Micro-scale MFCs have been studied for many years, yet the power density of them is orders of magnitude lower than that of macro-scale counterparts. In order to address this limitation, this work utilizes a ring-shape UME, which aims to enhance the diffusion of ions at the vicinity of micro-organisms, forming their biofilm on the UME. Areal and volumetric power densities of 7.72 W/m2 and 3,658 W/m3 are obtained, respectively; both are the highest ever reported among all MFCs to date, regardless of their sizes. In addition to the power densities, coulombic efficiency (CE) of 70 % and energy efficiencies of 20 % are marked, which makes the micro-scale MFC an attractive alternative in existing energy conversion portfolio.