We prepared amorphous carbon films on Si (100) substrates by using a magnetic-field-filtered plasma stream deposition system. Various samples with different sp3-bonded carbon fraction were obtained by changing the bias voltage applied to the substrates. We measured the ellipsometric spectra of various carbon film samples in the photon energy range of 2.0-5.0eV. We also measured the Raman spectra for comparison. Our results show that the ellipsometric spectra are dependent on the sp3 carbon fraction. We analysed the measured ellipsometric spectra by a simple method, and determined the sp3 carbon fraction semi-quantitatively. The results from the ellipsometry and the Raman spectroscopy show the same tendency of the sp3 carbon fraction as a function of bias voltage. We found that the spectroscopic ellipsometry is a relatively simple, non-destructive method to ewluate the sp3 carbon fraction of the amorphous carbon films.