Electric field modulations of magnetism in ferromagnetic metals are in general ascribed to the electric field effects on the orbitals. However, a direct observation of the electric-field-induced changes in the orbital states of a ferromagnetic metal is lacking. In this Rapid Communication, we demonstrate that an electric field applied to an ultrathin Co film can change the electron occupations according to the orbitals, combining x-ray absorption and x-ray magnetic circularly dichroism spectroscopies with the sum rules. Under a positive electric field, the dxz(yz)-occupied states decrease mainly via the modification of orbital hybridizations, while the dz2-occupied state increases. The changes in the dxz and dyz states dominantly alter the Curie temperature.