The excitation mechanism of the upstream low-frequency waves (0.01--0.05 Hz) is investigated based on the wave observations. We compared the observed wave frequencies with the frequencies expected from the ion (proton) beam-cyclotron instability process. We estimated the beam velocity from the solar wind parameters using Sonnerup's formula. Our analysis supports the hypothesis that these waves are excited by the resonant beam-cyclotron instability. For the wave observed in the deep foreshock region, however, we found it necessary to include the solar wind convection effect on the wave propagation.