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Microlensing of stars, e.g. in the Galactic bulge and Andromeda galaxy (M31),is among the most robust, powerful method to constrain primordial black holes(PBHs) that are a viable candidate of dark matter. If PBHs are in the massrange $M_{\rm PBH} \lower.5ex\hbox{$\; \buildrel < \over \sim \;$}10^{-10}M_\odot$, its Schwarzschild radius ($r_{\rm Sch}$) becomes comparablewith or shorter than optical wavelength ($\lambda)$ used in a microlensingsearch, and in this regime the wave optics effect on microlensing needs to betaken into account. For a lensing PBH with mass satisfying $r_{\rm Sch}\sim\lambda$, it causes a characteristic oscillatory feature in the microlensinglight curve, and it will give a smoking gun evidence of PBH if detected,because any astrophysical object cannot have such a tiny Schwarzschild radius.Even in a statistical study, e.g. constraining the abundance of PBHs from asystematic search of microlensing events for a sample of many source stars, thewave effect needs to be taken into account. We examine the impact of waveeffect on the PBH constraints obtained from the $r$-band (6210\AA) monitoringobservation of M31 stars in Niikura et al. (2019), and find that a finitesource size effect is dominant over the wave effect for PBHs in the mass range$M_{\rm PBH}\simeq[10^{-11},10^{-10}]M_\odot$. We also discuss that, if adenser-cadence (10~sec), $g$-band monitoring observation for a sample of whitedwarfs over a year timescale is available, it would allow one to explore thewave optics effect on microlensing light curve, if it occurs, or improve thePBH constraints in $M_{\rm PBH}\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}10^{-11}M_\odot$ even from a null detection.

On the wave optics effect on primordial black hole constraints from optical microlensing search