The present invention provides a thermostable duplex-specific nuclease (DSN) and a method for digesting a nucleic acid using said DSN, namely, a protein having a Brachyura-derived duplex-specific nuclease activity, a gene encoding for said protein, a recombinant vector comprising said gene and a transformed cell or transduced cell comprising said vector and a method for producing a protein having a duplex-specific nuclease activity, which comprises culturing said transformed cell or transduced cell using a medium and collecting the protein having a duplex-specific nuclease activity from the cultured mixture, a method for digesting a nucleic acid using said protein having duplex-specific nuclease activity, a method for detecting RNA using said DSN and a reagent kit to be used in the aforementioned methods.