  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 mass
range $M_{\rm PBH} \lower.5ex\hbox{$\; \buildrel < \over \sim \;$}
10^{-10}M_\odot$, its Schwarzschild radius ($r_{\rm Sch}$) becomes comparable
with or shorter than optical wavelength ($\lambda)$ used in a microlensing
search, and in this regime the wave optics effect on microlensing needs to be
taken into account. For a lensing PBH with mass satisfying $r_{\rm Sch}\sim
\lambda$, it causes a characteristic oscillatory feature in the microlensing
light 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 a
systematic search of microlensing events for a sample of many source stars, the
wave effect needs to be taken into account. We examine the impact of wave
effect on the PBH constraints obtained from the $r$-band (6210\AA) monitoring
observation of M31 stars in Niikura et al. (2019), and find that a finite
source 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 a
denser-cadence (10~sec), $g$-band monitoring observation for a sample of white
dwarfs over a year timescale is available, it would allow one to explore the
wave optics effect on microlensing light curve, if it occurs, or improve the
PBH constraints in $M_{\rm PBH}\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}
10^{-11}M_\odot$ even from a null detection.
