NOVELTY - Preparation of flomoxef sodium (VII) involves removing carboxyl protection group in 2-((difluoromethyl)sulfanyl)-N-((7R)-3-(((1-(2-hydroxyethyl)-1H-1,2,3,4-tetrazol-5-yl)sulfanyl)methyl)-7-methoxy-8-oxo-5-oxa-1-azabicyclo-oct-2-en-7-yl)acetamide compound (V) using fluorine-containing compound (VIII), obtaining (7R)-7-(2-((difluoromethyl)sulfanyl)acetamido)-3-(((1-(2-hydroxyethyl)-1H-tetrazol-5-yl)sulfanyl)methyl)-7-methoxy-8-oxo-5-oxa-1-azabicyclo-oct-2-ene-2-carboxylic acid (VI), and obtaining flomoxef sodium (VII) by reacting compound (VI) with sodium-containing compound (X).    USE - Preparation of flomoxef sodium (claimed).    ADVANTAGE - The method provides flomoxef sodium with high productivity and purity by simple, convenient, environmentally-friendly and reliable process in a short period of time.    DETAILED DESCRIPTION - Preparation of flomoxef sodium of formula (VII) involves reacting 3-(chloromethyl)-5-oxa-1-azabicyclo(4.2.0)oct-2-ene compound of formula (I) and methanol in presence of ceric ammonium nitrate, obtaining chloromethyl derivative of formula (II), reacting with tetrazole side chain, obtaining tetrazole derivative of formula (III), removing amino protection group and hydroxyl protection group under enzymatic hydrolysis process, obtaining 2-(1H-1,2,3,4-tetrazol-1-yl)ethan-1-ol compound of formula (IV), reacting with difluoromethyl mercaptoacetate, obtaining 2-((difluoromethyl)sulfanyl)-N-((7R)-3-(((1-(2-hydroxyethyl)-1H-1,2,3,4-tetrazol-5-yl)sulfanyl)methyl)-7-methoxy-8-oxo-5-oxa-1-azabicyclo(4.2.0)oct-2-en-7-yl)acetamide compound of formula (V), removing carboxyl protection group using fluorine-containing compound of formula (VIII), obtaining (7R)-7-(2-((difluoromethyl)sulfanyl)acetamido)-3-(((1-(2-hydroxyethyl)-1H-1,2,3,4-tetrazol-5-yl)sulfanyl)methyl)-7-methoxy-8-oxo-5-oxa-1-azabicyclo(4.2.0)oct-2-ene-2-carboxylic acid of formula (VI), and obtaining flomoxef sodium (VII) by reacting compound (VI) with sodium-containing compound of formula: R6CO2Na (X).    R1=acyl residue;    R2=carboxy-protecting group;    R3=hydroxy protecting group;    R4=O or S;    R5=1-benzotriazolyl or 2-benzothiazolyl;and    R6=alkyl.