STJs are promising X-ray detectors as high energy resolution spectrometers due to the small excitation energy to break the Cooper pairs to product detectable electrons. The expected energy resolution is about 5 eV for a 6 keV incident X-rays (see review by Kraus et al. and Esposito et al.). We have developed a large area (178 × 178μm
2) Nb/Al/AlOX/Al/Nb STJs (Kurakado et al. 1993) and series-connected STJs with a position resolution of 35μm for α particles (Kurakado 1997) at Nippon Steel Corporation. As a focal plane detector in future X-ray missions, we are developing STJs whose targert characteristics are; an energy resolution of 20 eV at 6keV, an effective area of 1 cm2, and position resolution of 100μm.