The intensity and spectral evolution of the bright X-ray nova GS 1124 -683, observed with the All Sky Monitor from January 5 to the end of August, 1991, are presented. The spectra are roughly represented by a two-component model consisting of a power law and a thermal (disk-blackbody or blackbody) component. In its rising stage, the power-law component was dominant, and the thermal component gradually increased with temperature and emission area. In the decay phase, the power-law component decreased rapidly, whereas the thermal component decayed exponentially with a time constant of 31.2 +/-0.1 days. Some implications of the accretion phenomenon during the X-ray nova activity are discussed.