1 Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi 442-8507, Japan

2 National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan

3 Nobeyama Radio Observatory, National Astronomical Observatory, Minami-saku, Nagano 384-1305, Japan

4 Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229-8510, Japan

It is observed in impulsive solar flares with the Hard X-ray Telescope (HXT) onboard Yohkoh that a hard X-ray source is located above the apex of a soft X-ray flaring loop. This observation suggests that the flare energy-release, probably magnetic reconnection, takes place not in the soft X-ray loop, but above the loop. It is important to derive the hard X-ray spectrum of this source accurately in order to understand how electrons are energized there. The above-the-looptop source was most clearly observed in the case of the 13 January 1992 flare. However, the count rate, especially in the HXT highest energy band (H-band; 53--93 keV), is too small in this event to synthesize high-quality images and to derive accurate spectra. In this situation, an X2.8-class impulsive flare, which occurred near (probably a little behind) the east limb at 8:17 UT on 18 August 1998, was observed with Yohkoh. The most outstanding characteristics of this event is that a hard X-ray source in the H-band is located 18,000 km high (higher than the apex of the soft X-ray flaring loop) above the limb around the peak time. For the first time, an above-the-looptop hard X-ray source was observed in such an intense impulsive flare. So we can derive accurate spectral information of three kinds of hard X-ray sources (the above-the-looptop source, footpoint sources, and looptop gradual source) in this event. In this paper, we discuss mainly the energy distribution of high energy electrons above the looptop from the result of spectral fitting, and the physical relationship of these three kinds of hard X-ray sources.