  Cosmic variance introduces significant uncertainties into galaxy number
density properties when surveying the high-z Universe with a small volume, such
uncertainties produce the field-to-field variance of galaxy number $\sigma_{g}$
in observational astronomy. This uncertainty significantly affects the
Luminosity Functions (LF) measurement of Lya Emitters (LAEs). For most previous
Lya LF studies, $\sigma_{g}$ is often adopted from predictions by cosmological
simulations, but barely confirmed by observations. Measuring cosmic variance
requires a huge sample over a large volume, exceeding the capabilities of most
astronomical instruments. In this study, we demonstrate an observational
approach for measuring the cosmic variance contribution for $z\approx2.2$ Lya
LFs. The LAE candidates are observed using narrowband and broadband of the
Subaru/Hyper Suprime-Cam (HSC), with 8 independent fields, making the total
survey area $\simeq11.62$deg$^2$ and a comoving volume of
$\simeq8.71\times10^6$Mpc$^3$. These eight fields are selected using the
project of MAMMOTH. We report a best-fit Schechter function with parameters
$\alpha=-1.75$ (fixed), $L_{Ly\alpha}^{*}=5.18_{-0.40}^{+0.43} \times
10^{42}$erg s$^{-1}$ and
$\phi_{Lya}^{*}=4.87_{-0.55}^{+0.54}\times10^{-4}$Mpc$^{-3}$ for the overall
Lya LFs. After clipping out the regions that can bias the cosmic variance
measurements, we calculate $\sigma_{g}$, by sampling LAEs within multiple
pointings assigned on the field image. We investigate the relation between
$\sigma_{g}$ and survey volume $V$, and fit a simple power law:
$\sigma_g=k\times(\frac{V_{\rm eff}}{10^5 {\rm Mpc}^3})^{\beta}$. We find
best-fit values of $-1.209_{-0.106}^{+0.106}$ for $\beta$ and
$0.986_{-0.100}^{+0.108}$ for k. We compare our measurements with predictions
from simulations and find that the cosmic variance of LAEs might be larger than
that of general star-forming galaxies.
