The present invention provides a 2-OST mutant exhibiting a high activity. Specifically, the present invention provides a 2-O-sulfation enzyme mutant, having a substitution of a leucine residue at position 321 with a basic amino acid residue in any one amino acid sequence of: (a) the amino acid sequence of SEQ ID NO: 2; (b) an amino acid sequence comprising one or several amino acid substitutions, deletions, insertions, or additions in the amino acid sequence of SEQ ID NO: 2; (c) an amino acid sequence having 90% or more identity to the amino acid sequence of SEQ ID NO: 2; (d) the amino acid sequence consisting of amino acid residues at positions 69 to 356 in the amino acid sequence of SEQ ID NO: 2; (e) an amino acid sequence comprising one or several amino acid substitutions, deletions, insertions, or additions in the amino acid sequence consisting of amino acid residues at positions 69 to 356 in the amino acid sequence of SEQ ID NO: 2; (f) an amino acid sequence having 90% or more identity to the amino acid sequence consisting of amino acid residues at positions 69 to 356 in the amino acid sequence of SEQ ID NO: 2; and having a 2-O-sulfate transfer activity.