The present invention provides a method for producing a heparosan compound having an isomerized hexuronic acid residue and a method for producing a heparan sulfate with improved C5-epimerization efficiency. Specifically, the present invention provides a method for producing a heparosan compound having an isomerized hexuronic acid residue, said method comprising producing the heparosan compound having the isomerized hexuronic acid residue from a heparosan compound in the presence of a protein selected from the group of consisting of the following (A) to (F): (A) a protein comprising the amino acid sequence of SEQ ID No:2; (B) a protein which comprises an amino acid sequence having 90% or more homology to the amino acid sequence of SEQ ID No:2 and has a D-glucuronyl C5-epimerase substituted, added or inserted amino acid residues in the amino acid sequence of SEQ ID No:2 and has a D-glucuronyl C5-epimerase activity; (D) a protein comprising the amino acid sequence of SEQ ID NO:5; (E) a protein which comprises an amino acid sequence having 90% or more homology to the amino acid sequence of SEQ ID No:5 and has a D-glucuronyl C5-epimerase activity; and (F) a protein which comprises an amino acid sequence having one or several deleted, substituted, added or inserted amino acid residues in the amino acid sequence of SEQ ID No:5 and has a D-glucuronyl C5-epimerase activity.