The present invention relates to a thermophilic arabinose isomerase and a method of manufacturing tagatose using the same, and more precisely, a gene encoding arabinose isomerase originating from the thermophile Geobacillus stearothermophilus DSM22, a recombinant expression vector containing the gene, a method of preparing a food grade thermophilic arabinose isomerase from the recombinant GRAS (Generally Recognized As Safe) strain transformed with the said expression vector, and a method of preparing tagatose from galactose using the said enzyme.