We have prepared microemulsions consisting of water/[40 wt % polyoxyethylene (20 mol) glycerin isostearate (abbreviated as POE-GIS) + 60 wt % random copolymer of polyoxyethylene (POE, 38 mol)/polyoxypropylene (POP, 10 mol) pentaerythritol tetramethyl ether (abbreviated as PEPTME (38/10)}]/[polyoxyethylene (POE, 19 mol)/ polyoxypropylene (POP, 19 mol) polydimethylsiloxane copolymer (abbreviated as POE/POP-PDMS)] and water/[40 wt % POE-GIS + 60 wt % PEPTME (38/10)]/[95 wt % POE/POP-PDMS + 5 wt % oleic acid (abbreviated as OA)] systems and characterized them with optical observation, rheometry, and freeze-fracture transmission electron microscopy (FF-TEM) images. Bicontinuous and droplet-type O/W (oil-in-water) microemulsions are formed depending on the volume fraction of water. The bicontinuous structure observed in the oil-rich region, upon successive dilution with water, is transformed into a droplet-type microemulsion without phase separation. The prepared droplet-type microemulsion containing polymeric silicone and random copolymer PEPTME (38/10) as a cosurfactant in the water-rich region has potential applications in cosmetics.