We demonstrate experimentally the hyperfine-interaction control by an electric field, which is the operating principle of the addressable qubit operation in a silicon-based solid-state quantum computer in a new quantum computer system, a magnetic crystal. The transferred hyperfine field at a F(-) nucleus caused by neighboring Mn(2+) electron spins in an antiferromagnetic MnF(2) single crystal was measured by (19)F nuclear magnetic resonance (NMR) with an external electric field applied along the [110] crystal direction. The electric field splits the (19)F NMR peak into two resolved lines that come from the F nuclei located at geometrically equivalent sites. A line splitting of 56 kHz was achieved at an electric field of 3.4 V/mu m. One of the F(-) nuclear spins could be flipped selectively by a composite radio-frequency pulse while leaving the other unchanged, thereby demonstrating qubit addressing via electric field control of the hyperfine interaction.