Epitaxial a-axis YBa2Cu3O7–δ thin films have been grown on SrTiO3 (100) by on-axis 94.92 MHz magnetron sputtering using the YBa2Cu5Ox target at a self-bias voltage of −50 V and YBa3Cu7Ox target at a cathode voltage of −100 V adjusted by externally applying a negative dc voltage. Good crystalline quality of the films sputtered at −50 V was confirmed from a full width at half maximum, δω, of 0.04° through the (100) peak by x-ray diffraction and a minimum yield, Xmin, of 4.8% by Rutherford backscattering channeling. Those films, however, have not shown superconductivity. On the other hand, the films sputtered at −100 V were characterized by a δω of 0.12° and a Xmin of 4.3%. Superconductivity occurs in the films only by shifting a self-bias voltage from −50 V on the YBa2Cu5Ox target to −100 V on the YBa3Cu7Ox target. The zero resistance temperature is 33 K at most.