  We report the discovery of possible periodic X-ray dips in a pulsating
ultraluminous X-ray source, M51 ULX-7, with the archival Changra observations.
With ~20 days of monitoring in the superorbital descending state, we discovered
three dips with separations of ~2 and ~8 days via the Bayesian block technique.
A phase-dispersion minimization and a $\chi^2$ test suggest that the dip is
likely recurrent with a period of ~2 days, consistent with the orbital period
of M51 ULX-7. We interpret the dip as an obscuring of the emission from the
pulsar by the vertical structure on the stream-disk interaction region or the
atmosphere of the companion star. Both interpretations suggest the viewing
angle to be ~60 degrees. Given that the magnetic field of M51 ULX-7 is
moderately high, $B\sim10^{13}$ G, low geometric beaming with $b\lesssim1/2$ is
sufficient to explain the observed flux and the presence of dips. Obscuration
of the stellar wind remains an alternative possible origin and further
monitoring of the dips will be required.
