An energy spectrum of electrons from 180 to 550 keV precipitating into the dayside polar ionosphere was observed under a geomagnetically quiet condition (AE ≤ 100 nT, Kp = 1-). The observation was carried out at 73–184 km altitudes by the HEP instrument onboard the RockSat-XN sounding rocket that has been launched from Andøya, Norway. The observed energy spectrum of precipitating electrons follows a power law of −4.9 ± 0.4 and the electron flux does not vary much over the observation period (∼274.4 s). A nearby ground-based VLF receiver observation at Lovozero, Russia shows the presence of whistler-mode wave activities during the rocket observation. A few minutes before the RockSat-XN observation, POES18/MEPED observed precipitating electrons, which also suggest whistler-mode chorus wave activities at the location close to the rocket trajectory. A test-particle simulation for wave-particle interactions was carried out using the data of the Arase satellite as the initial condition which was located on the duskside. The result of the simulation shows that whistler-mode waves can resonate with sub-relativistic electrons at high latitudes. These results suggest that the precipitation observed by RockSat-XN is likely to be caused by the wave-particle interactions between whistler-mode waves and sub-relativistic electrons.