NOVELTY - The carbon material is pre-treated by heating and the pre-treated carbon material is oxidized using an oxidant. Thus, method for producing electrode active material is carried out.    USE - For method for producing electrode active material for electrode used for energy storage device (both claimed) such as electrochemical double layer capacitor, hybrid capacitor, lithium secondary battery, solar cell and fuel cell used as power source for back-up power source for portable communications device and home electronics such as cellular phones, camera and audio and video devices and main or auxiliary power sources for uninterruptible power supply used for hybrid electrical vehicle.    ADVANTAGE - The energy storage device is ecofriendly, economical and has excellent charging and discharging characteristic and durability.    DETAILED DESCRIPTION - The carbon material is soft carbon. The soft carbon is formed of one or more materials chosen from an aliphatic polymer compound such as vinyl chloride resin and polyacrylonitrile, aromatic polymer compound such as mesophase pitch and polyimide, coal tar pitch, petroleum coke, coal tar coke, mesocarbon microbeads, and mesophase pitch spinning fiber. The oxidant is chosen from nitric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, arsenic acid, selenic acid, perchloric acid, trifluoro acetic acid, trifluoro bromoacetate, fluoro sulfuric acid, periodic acid, potassium permanganate, sodium nitrate, potassium chlorate, sodium chlorate, ammonium chlorate, silver chlorate, chloric acid, sodium perchlorate, ammonium perchlorate, chromic oxide, ammonium persulfate, lead oxide, manganese oxide, arsenic oxide, sodium oxide, hydrogen peroxide, dinitrogen pentoxide, ethanol and methanol. The electrode contains binder (80-95 weight parts) with respect to 100 weight parts of the active material. The electrolytic solution in the energy storage device contains quaternary ammonium salt, quaternary imidazolium salt, quaternary pyridinium salt, quaternary hoshonium salt and/or lithium salt.