NOVELTY - A method for estimating cetane value of oil involves measuring the styrene concentration of oil obtained by cooling the collected thermolyzed gas of waste plastic. The amount of polystyrene in the waste plastic is determined based on styrene concentration, and the cetane value is estimated.    USE - For estimation of cetane value of oil obtained from waste plastic, which is used as fuel in diesel engine generator in electric power generating device (claimed).    ADVANTAGE - The method is simple and the cetane value of the oil is adjusted efficiently. The direct coupling of diesel engine generator with the waste plastic oil formation apparatus is enabled.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following: (i) Formation of waste plastic oil involving thermolysis of waste plastic using thermal decomposition furnace. The thermolyzed gas ejected from the thermal decomposition furnace is cooled to fixed temperature (I) and a part of gas is condensed and recirculated to the thermal decomposition furnace. The remaining gas is cooled to fixed temperature (II) which is lower than the temperature (I), to form oil. The concentration of styrene in oil is measured and the amount of polystyrene is determined. The cetane value of the oil is estimated and the temperature (I) is increased if the estimated cetane value is lower than preset value or a paraffin oil is mixed with the oil which is obtained during the process; and (ii) Waste plastic oil formation apparatus comprising thermal decomposition furnace, cooler (300) for cooling thermolyzed gas from thermal decomposition furnace, and a recirculation tank (400) for circulating the heavy liquid and gas component which are condensed in the cooler, and recirculates heavy liquid to the furnace. Cooler (500) cools gas coming from the recirculation tank. A vapor-liquid separation tank (600) separates oil and gas which are condensed in the cooler (500). Fuel storage tank for storing oil separated by the vapor-liquid separation tank is contained. The amount of polystyrene in the waste plastic is determined from the measured concentration of styrene in oil. If the cetane value is lower than the preset value the temperature (I) is increased or paraffin oil is mixed in the fuel oil storage tank, based on a control apparatus.    DESCRIPTION OF DRAWING(S) - The figure shows outline of waste plastic oil formation-electrical power generating device.    Coolers (300,500)    Recirculation tank (400)    Vapor-liquid separation tank (600)