A crossed beam technique is used to measure ionization cross sections of metallic atoms. Relative values of cross sections of single, double and triple ionization of magnesium have been successfully measured over the 0-700 eV range with an estimated accuracy of 11%, 19% and 25%, respectively. Absolute values of cross sections have been obtained by normalization to a theoretical value at high electron energy. The theoretical value is calculated by using the generalized oscillator strength approximation. Results are compared to previously published values and, for single ionization in particular, a comparison with theoretical cross sections is performed.