Magnetic field homogeneity and stability of a Bitter type resistive magnet of National Institute for Materials Science (NIMS) have been examined at fields up to 23.5 T by means of NMR. The field homogeneity is 100 +/- 10 ppm over a 5 mm DSV (diameter of spherical volume), being independent of field strength. The field instability observed at 23.5 T contains, at least, two contributions; 1) a periodic oscillation of frequency 50 +/- 1 Hz and amplitude 100 +/- 25 ppm, and 2) a drift at a typical rate of 50 +/- 20 ppm/h. The magnet power supply seems to be primarily responsible for both instabilities.