The modern biwas used in Japanese music can be classified into two groups, ``Satsuma'' biwa and ``Chikuzen'' biwa. In this paper, the structural difference between Chinese biwas (PiPa) and Satsuma biwas [especially the fret height and ``torikuchi'' (nut), related to ``sawari''], which have the same origin, are discussed by one of the authors, Y. Tohnai. Tohnai is a skilled player and a researcher of modern biwas. Three Satsuma biwas, with approximately the same size and shape but from different generations, were selected from Tohnai's collection. The open strings of all three were played three times by Tohnai in an anechoic chamber; the tones were recorded on DAT and analyzed using a sound spectrograph and FFT analyzer. The most prominent result, common to all the tones, is that the sound-pressure level of the fundamental tone and lower overtones is very low compared with 6--12th overtones (from -40 to -30 dB). Psychological experiments must be carried out in order to study its influence on the sensation of ``pitch.''