This study was conducted to investigate the yield response of soybean to drought stress in 1984 and 1986 at the experiment field of the National Academy of Agricultural Science using experiment plots with different soil water tension and fertilizer levels. The average yield response factor (YRF) of soybean to evapotranspiration (ET) calculated as [(Ya/Ym)/(ETa/ETm)], where Ya, average yield; Ym, maximum yield; ETa, average ET; and ETm, maximum ET, was 0.91 with the range from 0.74 to 1.16. Relationship between yield index (YI=[Ya/Ym]) and evapotranspiration index (ETI=[ETa/PET]) was $YI=0.87{cdot}(ETI)+0.09$. Relationship between YI and the maximum soil water tension (Hmax) was $YI=1.23-0.23{cdot}{log}$ (Hmax). Relationship between YI and the days of drought stressed (Dr) was $YI=0.877{cdot}{exp}$ ($-0.01{cdot}Dr$). The relation between YI and fertilizer level (F) was $YI=-0.21{cdot}F2+0.36{cdot}F+0.33$, under very serious drought condition as the maximum soil water tension was 0.3 MPa.