National Radio Observatory, Nobeyama, Minamisaku, 384-1305, Japan

Solar flares are one of the most spectacular phenomena taking place in the solar corona. Since hard X-rays emitted from footpoint sources are considered as the most direct signature of accelerated electrons, it is crucially important to exactly know the hard X-ray spectrum for unveiling the electron acceleration mechanism. Before the Yohkoh/HXT the hard X-ray spectrum had been measured without relevant spatial resolution, and hence the result is still confusing; some solar flares show a broken power-law spectrum and some other show a mixture of super-hot thermal spectrum and nonthermal tail spectrum. Although the Yohkoh/HXT has only four energy bands, it is the first instrument capable of spatially resolving footpoint sources from other sources. Thus we can derive a hard X-ray spectrum of footpoint sources separated from other components. I have analyzed hard X-ray flares in which the footpoint sources dominate throughout the four energy bands. A comparison of observed four-band photon counts with the Bremsstrahlung emission calculated on the assumption of a single power-law electron spectrum reveals a deficiency of observed photon counts at the low-energy side, which is suggestive that the electron spectrum has a low-energy cutoff above 30 keV.