NOVELTY - Manufacture of engine fuel comprises newly introducing a fresh reducing agent and a fresh polymerization inhibitor both having higher activities and for simultaneously introducing ozone, into the filtrate, to crack the oil/fat component while subsequently stirring the component; and repeating the treatment steps for cracking the oil/fat component in a manner to conduct such a treatment step totally two more times.    USE - For manufacture of engine fuel.    ADVANTAGE - The invention is capable of producing engine fuels of higher performances in addition to the epoch-making ability. It allows to more effectively and suitably adjusts an oxidation cracking reaction to be caused by ozone, to thus efficiently obtaining engine fuels of superior quality.    DETAILED DESCRIPTION - Manufacture of engine fuel comprises:    (a) filtering a higher-boiling-point vegetable oil/fat;    (b) introducing ozone, reducing agent and polymerization inhibitor into the higher-boiling-point oil/fat to cause the oil/fat component and the ozone to react with each other while adjusting the reactivity in the presence of the reducing agent and polymerization inhibitor to thus crack the oil/fat component;    (c) filtering the oil/fat to remove the solid components including the reducing agent and the polymerization inhibitor having been deteriorated by the cracking reaction;    (d) newly introducing a fresh reducing agent and a fresh polymerization inhibitor both having higher activities and for simultaneously introducing ozone, into the filtrate, to crack the oil/fat component while subsequently stirring the component; and    (e) repeating the treatment steps for cracking the oil/fat component in a manner to conduct such a treatment step totally two more times while interposing the filtering step between the treatment steps, thus reform the oil/fat into a lower oil while delivering an antifreezing agent into the lower-boiling-point oil.    INDEPENDENT CLAIMS are also included for:    (a) an apparatus for manufacturing engine fuel comprising a reaction drum into which the higher-boiling-point oil/fat is introduced; and stirring mechanism within the reaction drum and having rotating propeller plate for revolving a reaction liquid; and a baffle provided at an inner periphery of the reaction drum and having a vertical plate with a widthwise portion protruded from an inner peripheral wall surface of the reaction drum toward the center of the reaction drum, and the baffle is formed with saw-toothed cutouts at the protruded end; and the reaction drum is provided with a delivering port for delivering a reducing agent and a polymerization inhibitor into the reaction drum; and    (b) a plant for manufacturing engine fuel, comprising a starting-material tank for storing oil/fat; a filtering device for filtering out solid components of the oil/fat in the starting-material tank; a first reaction vessel (2) for conducting first cracking by reacting ozone with the oil/fat filtered by the filtering device; an oil/water separation device for separating the water content involved in the cracking reaction, from the oil/fat; a filtering medium charging vessel (7) for delivering a filtering medium into the oil/fat phase from which the water phase component has been excluded by the oil/water separation device; a first filtering device (8) for conducting first filtering of the oil/fat into which the filtering medium has been delivered; a second reaction vessel (9) for reacting ozone with the filtered oil/fat, to thus conduct second cracking; a second filtering device (10) for conducting second filtering of the oil/fat subjected to the second cracking; an adding device for adding an additive into the oil/fat subjected to the second filtering; and an impurity adsorbing vessel and a filtering vessel for cooperatively purifying the water phase component from which the oil/fat phase has been excluded by the oil/water separation device; where each of the first reaction vessel and the second reaction vessel is provided with a delivering port for delivering a reducing agent and a polymerization inhibitor into the reaction vessel.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a manufacturing plant.    First reaction vessel (2)    Filtering medium charging vessel (7)    First filtering device (8)    Second reaction vessel (9)    Second filtering device (10)