Wind turbine noise is evaluated, in accordance with a manual published by the Japanese Ministry of the Environment, by the difference between the residual sound level and the wind turbine sound level, averaged over at least three days within the effective wind speed range. Variations in residual sound levels from varying wind speeds during measurement caused errors in the evaluated values. We investigated statistically the relation between wind speed and residual sound level. The measurement location and timing caused some changes, but a statistically significant residual sound level difference was found, with a wind speed difference at least 1-3 m/s within the effective wind speed range. The increase in residual sound was approximately 0.8-1.4 dB for a 1.0 m/s increase in wind speed. The 90% range of scattering in the daytime and nighttime average wind speed under evaluation conditions, obtained from wind speed data gathered over a period of one year at three locations, was 1.9-9.0 m/s. When the wind speed difference, within the three-day average to be evaluated, was in the top 5%, the residual sound level was significantly different only in 20-50% of cases, and sound levels differed within a range of about 7 dB.