ABSTRACT: We have performed 3 sets of one-month flux density monitoring of the compact synchrotron source, Sgr A* at 100 GHz using the Nobeyama Millimeter Array (NMA) in 1996-1998. The NMA observations allow us to spatially separate Sgr A* from the surrounding thermal component. All the observations were made with the same array configuration. We detected a flare of Sgr A* in March of 1998, although there is no such significant variability during other observing months. The flux density of Sgr A* at 100 GHz flared by 200% in a week and decreased to the mean flux density within two weeks. No remakable change is seen in NRAO monitoring data at 2 and 8 GHz during the flare. The rapid flux density variability at mm wavelengths was observed previously but is still controversial. The entire time profile of a flare is clearly displayed in our data. Some implications of the results are discussed.