NOVELTY - A biodiesel oil is prepared by calcining sodium silicate; pulverizing to obtain sodium silicate catalyst powder; mixing catalyst, lipid and short chain alcohol in reactor; carrying out etherifying reaction; stirring to obtain crude biodiesel oil product; subjecting to centrifuge; separating upper phase of centrifuged liquid; distilling to obtain refined diesel oil and to recover alcohol; separating lower phase which is glycerol; recovering glycerol using catalyst; and separating catalyst for recycling.    USE - A preparation method of biodiesel oil.    ADVANTAGE - The method can recover catalyst easily thus simplifying production, recycles catalyst thus saving production cost, prepares an improved diesel oil product, and has a minimized discharged of contamination.    DETAILED DESCRIPTION - A biodiesel oil is prepared:    (A) calcining sodium silicate at 200-900 degrees C for 1-5 hour;    (B) pulverizing to obtain sodium silicate catalyst powder having particle size of 50-200 mesh sieve size;    (C) mixing catalyst (1-5wt.%), lipid and short chain alcohol in reactor at 60-90 degrees C;    (D) carrying out etherifying reaction at pre-determined mole ratio;    (E) stirring at 100-300 rpm to obtain crude biodiesel oil product;    (F) subjecting to centrifuge at a speed of 3000 rpm for 6-12 hours;    (G) separating the upper phase of the centrifuged liquid;    (H) distilling to obtain refined diesel oil and to recover the alcohol;    (I) separating lower phase which is glycerol;    (J) recovering the glycerol using the catalyst; and    (K) separating the catalyst.    The catalyst is recovered by washing separated catalyst from the biodiesel oil with hot water at 60-90 degrees C or organic solvent which is alcohol or alkane compound; heating the catalyst to remove water; adding alkali solution; regenerating catalyst by calcining at 80-150 degrees C; drying calcined catalyst at 300-900 degrees C for 1-5 (preferably 1-3) hours; adding sodium salt; and recycling the catalyst to be used for preparing biodiesel oil. The alkali solution is sodium hydroxide or sodium methylate. The sodium salt is sodium nitrate, sodium percarbonate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, or trisodium phosphate.