Considerable attention has been paid to protecting native biodiversity from the threat of cervid overpopulation, with shared challenges among various countries. Sika deer (Cervus nippon) is a typical cervid, which has recently expanded its distribution within its native ranges (i.e. far eastern Asia and Russia), and several introduced areas in Europe, USA, and New Zealand. With a growing demand for precautionary measures against such threats, which are mainly associated with males in the initial invasive stage, a new approach has been introduced in this study, passive acoustic monitoring (PAM), to detect males with a high sensitivity using their howl during the rut. Here, we verified the feasibility of PAM by comparing it with existing methods using spotlights and camera traps at five sites with different deer densities in the eastern mainland of Japan. For the verification experiments, we set unattended sound recorders to evaluate time and site variations in howl frequency, and measured the detection range of the howl. Consequently, PAM successfully detected males, even at sites with extremely low deer density, where the existing methods were unsuccessful. The detection zone of PAM was around 6 ha in defoliated forests, which was > 200 times higher than that for commonly used camera traps. Thus, the current findings suggest that bioacoustic methods could be beneficial for monitoring low-density populations, which are observed in the initial stage of deer invasion or in the ex-post stage following population regulation.