Measurements of spectra in the wavelength range from vacuum ultraviolet (VUV) to soft X-ray are important means to diagnose impurities in magnetically confined plasmas used in fusion plasmas such as a GAMMA 10 plasma. Recently, a space- and time-resolving flat-field grazing-incidence VUV spectrograph was constructed for the simultaneous observation of spatial, temporal and spectral distributions of plasma radiation in the wavelength range of 150-1050 Å. Absolute calibration experiments of the space- and time-resolving VUV spectrograph in the wavelength range of 450-1050 Å were performed for the first time under both S and P polarized light conditions at beamline 11C in the Photon Factory at the High Energy Accelerator Research Organization. Thus, we can obtain radial profiles of the absolute emissions from the impurities by using Abel inversion, and estimate the density of impurity ions such as oxygen, carbon and so on. From the total impurity ion densities, we can estimate Zeff. During the formation of plug potential by ECRH, the highly-ionized impurities increased as a result of rising of electron temperature. The Zeff with plug potential is larger than that without plug potential.