In this study, the microscopic origins of the voltage-controlled magnetic anisotropy (VCMA) in 3d-ferromagnetic metals are revealed. Using in situ x-ray fluorescence spectroscopy that provides a high quantum efficiency, electric-field-induced changes in orbital magnetic moment and magnetic dipole Tz terms in ultrathin Co films are demonstrated. An orbital magnetic moment difference of 0.013μB was generated in the presence of electric fields of ±0.2V/nm. The VCMA of Co was properly estimated by the induced change in orbital magnetic moment, according to the perturbation theory model. The induced change in the magnetic dipole Tz term only slightly contributed to the VCMA in 3d-ferromagnetic metals.