We are developing of adiabatic demagnetization refrigerator for X-ray microcalorimeter operation. By improving thermal environment in the cryostat, it achieved the lowest temperature of 80 mK, temperature stability of 9 µK rms at 100 mK, and a hold time of 7 hours at 100 mK. The excess heat load to the saltpill decreased from 10 µW to 3 µW. Using this system, we demonstrated a measurement of R-T curve of a TES microcalorimeter and φ-V curve of a SQUID.