We have developed a microstrip gas chamber (MSGC) by using multi-chip technology which enables high-density assembly of bare LSI chips on a silicon board. Our MSGC was operated steadily with approximately 103gain more than one week. An energy resolution of 15% (FWHM) for 5.9 keV X ray of55Fe was obtained. With a very thin polyimide substrate of 16 μm thickness, two interesting phenomena were observed; one is a strong dependence of gains on the back plane potential, and the other is little time variation of gains. A new type of MSGC with a guarding mask of a thin polyimide layer on the cathode edges has been examined to reduce incidental electrical discharges between anode and cathode strips. Furthermore, a new approach to reduce the resistivity of the substrate has been examined. By these approaches, the stability of the high gain operation of approximately 104has been drastically improved. In addition, we discuss the possibility of the application of MSGC to the coded mask x-ray imaging detector for astrophysics.