The present application relates to a modified isoprene synthase that has an isoprene synthetic activity and has at least one mutation of an amino acid residue in the amino acid sequence of SEQ ID NO: 4, an amino acid sequence having one or several amino acid substitutions, deletions, insertions or additions in the amino acid sequence of SEQ ID NO: 4, or an amino acid sequence having 90% or more identity to the amino acid sequence of SEQ ID NO: 4. The modified isoprene synthase is useful for preparing isoprene monomers in improved yields.