To provide a new heat-resistant protein having aconitic acid hydratase activity.

The new protein having aconitic acid hydratase activity comprises a specific amino acid sequence. This protein, which is a hyperthermophilic archaebacterium, is obtained by cloning a gene estimated to encode a protein having aconitic acid hydratase activity from the gene sequence of Sulfolobus tokodaii strain 7(JCM10545), a kind of aerobic thermoacidophilic crenarchaeon, followed by expression using Escherichia coli.