To provide a new approach to the production of restriction endonucleases.

Provided is a new method for the overexpression of these enzymes, which comprises transforming a host cell with an expression vector containing a DNA encoding the restriction endonuclease and an expression vector of a methylase capable of protecting the host cell DNA against the restriction endonuclease. The methylase vector is compatible with the restriction endonuclease expression vector. Preferably, the expression vector is a T7 expression vector, and the host cell is transformed with a third compatible vector which regulates the expression vector.