The present invention provides a heat-stable enzyme having a DNA helicase activity, and preferably a structure-specific endonuclease activity. This enzyme can be obtained by cloning a gene encoding the enzyme from chromosomal DNA of a hyperthermophile, preferably a sulfur-metabolizing thermophilic archaebacteria, incorporating the gene into an expression vector, introducing the vector into a host such as Escherichia coli in the usual manner to create a transformant, and culturing the transformant.