The spectral characteristics of the plasma radiation of a gas Z-pinch with arise time of 1 mu s and a current amplitude of 250kA have been measured. The measurements were performed in the range of X-ray energy quanta from 30eV to 600eV. A spectrograph with a transmission grating and a polychromator on the basis of multilayer X-ray mirrors served as the diagnostic device. Absolutely calibrated semiconducting detectors AXUV-5 were used as radiation detectors in the polychromator. The full pinch radiation loss was measured with a thermocouple calorimeter. The radiation of He-, Li- and Be-like ions has been shown to dominate at the stage of maximum compression of the nitrogen Z-pinch. The results of the spectral measurements were employed for the absolute calibration of various X-ray detectors. Some of these calibration results are presented in this paper.