A clear later phase ~80 s after the direct P wave is observed in most of individual seismograms recorded by a short-period seismometer network in Japan (J-array) from a cluster of deep earthquakes that occurred at the northern Mariana subduction zone. This phase (1) shows a P wave particle motion, (2) arrives later from earthquakes with shallower focal depths, (3) has a steeper incident angle than that of P wave, and (4) shows a deviation of a few degrees in the arrival azimuth from that of P wave. We interpret it as an S-to-P converted wave which takes off downward from the source and is reflected at a velocity discontinuity (reflector) below the earthquakes. Travel time inversion shows that the seismic reflector dips southwest by ~20¿ at 24.25¿N, 144.75¿E, and at a depth of 1115 km with a lateral extension at least 100 ¿ 100 km. The location corresponds to the lower edge of a high-velocity anomaly in global tomographic models. Amplitude and waveform analyses suggest a decrease of S wave velocity by 2--6% and an increase of density by 2--9% within the reflector. There is almost no difference in P wave velocity (<1%) between the reflector and the surrounding mantle. The estimated thickness of the reflector is ~12 km. These observations indicate that the observed seismic structure is more likely to be a chemical reservoir rather than a purely thermal anomaly. The seismic reflector might be a piece of subducted oceanic crust, as suggested by a previous study. It also could be related to the break down of the D phase of dense hydrous magnesium silicates at midmantle pressure condition reported by recent mineral physics studies. Both scenarios imply that mechanical/chemical segregation might occur within the subducted slab at midmantle condition.