Observation and cutting of DNA molecules at intended positions permit several new experimental methods that are completely different from conventional molecular biology methods; therefore several cutting methods have been proposed and studied. In this paper, a new cutting method for a DNA molecule by localizing the activity of a restriction enzyme is presented. Since most restriction enzymes require magnesium ions for their activation, local restriction enzyme activity can be controlled by the local concentration of magnesium ions. Applying a direct current (dc) voltage to a needle electrode of metallic magnesium made it possible to control the local magnesium ion concentration at the tip of the needle. The restriction enzyme was activated only when magnesium ions were electrochemically supplied.