The present invention relates to a method for producing tagatose using soy oligosaccharide or soluble sugar solution containing the same, more precisely, a method for producing tagatose comprising the following steps hydrolyzing soy oligosaccharide by using ±-galactosidase selectively producing tagatose continuously by enzymatic isomerization of galactose obtained from the hydrolysate separating the produced tagatose by chromatography and recycling the non-reacted materials.