  We study the formation of the first galaxies in overdense regions modelled by
the FORmation and EVolution of galaxies in Extremely overdense Regions
motivated by SSA22 (FOREVER22) simulation project. Our simulations successfully
reproduce the star formation rates and the $M_{\rm UV}-M_{\rm star}$ relations
of candidate galaxies at $z \sim 10-14$ observed by the James Webb Space
Telescope (JWST). We suggest that the observed galaxies are hosted by
dark-matter haloes with $M_{\rm h} \gtrsim 10^{10}~{\rm M_{\odot}}$ and are in
short-period starburst phases. On the other hand, even simulated massive
galaxies in overdense regions cannot reproduce the intense star formation rates
and the large stellar masses of observed candidates at $z \sim 16$. Also, we
show that the contribution of population III stars to the UV flux decreases as
the stellar mass increases and it is a few percent for galaxies with $M_{\rm
star} \sim 10^{7}~{\rm M_{\odot}}$. Therefore, a part of the observed flux by
JWST could be the light from population III stars. Our simulations suggest that
the UV flux can be dominated by population III stars and the UV-slope shows
$\beta \lesssim -3$ if future observations would reach galaxies with $M_{\rm
stars} \sim 10^{5}~{\rm M_{\odot}}$ at $z \sim 20$ of which the mass fraction
of population III stars can be greater than 10 percent.
