Ocean swell activities excite body‐wave microseisms that contain information on the Earth's internal structure. Although seismic interferometry is feasible for exploring structures, it faces the problem of spurious phases stemming from an inhomogeneous source distribution. This paper proposes a new method for inferring seismic discontinuity structures beneath receivers using body‐wave microseisms. This method considers the excitation sources of body‐wave microseisms to be spatially localized and persistent over time. To detect the P‐s conversion beneath the receivers, we generalize the receiver function analysis for earthquakes to body‐wave microseisms. The resultant receiver functions are migrated to the depth section. The detected 410‐ and 660‐km mantle discontinuities are consistent with the results obtained using earthquakes, thereby demonstrating the feasibility of our method for exploring deep‐earth interiors. This study is a significant step toward body‐wave exploration considering the sources of P‐wave microseisms to be isolated events. Plain Language Summary The ocean waves excite persistent and random ground motions called microseisms. Since this excitation is independent of seismic activities, this wavefield has information about seismic structures that earthquakes never have. For the deep structure, such as the mantle and core, body‐wave microseisms are more suitable than surface‐wave microseisms because body‐wave microseisms have better sensitivity. Previous studies using body‐wave microseisms mainly adopted the cross‐correlation analysis known as seismic interferometry. This method assumes that the microseisms are excited everywhere. However, the inhomogeneous source distribution of body‐wave microseisms causes artifacts for exploration by seismic interferometry. We developed a new method which circumvents this problem. Assuming that the body‐wave microseisms are spatially isolated, this method extracted the P‐s converted waves beneath receivers from body‐wave microseisms. The 3‐Dimensional imaging result of extracted P‐s converted waves shows both 410‐ and 660‐km mantle discontinuities, consistent with results using earthquakes. This study shows the potential of body‐wave microseisms for exploring the deep earth structure. Key Points The P‐S waves at mantle discontinuities were extracted from the ambient noise excited by the ocean swells We developed the source deconvolution method to generalize a receiver function method to P‐wave microseisms The migration result of P‐S waves was consistent with previous studies, showing the potential of P‐wave microseisms to seismic structures