X-ray absorption spectroscopy (XAS) in the soft X-ray region is important for materials analysis of compound semiconductors or catalysts, for which light elements are important to improve their performances. Superconducting tunnel junction (STJ) detector is promising for XAS in the soft X-ray region, because of its high sensitivity and high element selectivity compared to conventional spectrometers using semiconductor detectors or gratings. We constructed a soft X-ray spectrometer using a 100-pixel STJ array detector (Fig. 1, Fig. 2). To demonstrate the high sensitivity and high element selectivity of the STJ spectrometer, we observed the fluorescence of nitrogen from an SiC sample with nitrogen density of 300 ppm, for which it was impossible to detect the N-Kα line separating from the C-Kα line using conventional spectrometers. We successively observed the N-K edge XAS of the sample, separating the weak N-Kα line from the prominent C-Kα line (Fig. 3). The STJ spectrometer may be a powerful tool to improve performances of materials which require a low density light element dopant.