  We report the mass determination of TOI-519 b, a transiting substellar object
around a mid-M dwarf. We carried out radial velocity measurements using Subaru
/ InfraRed Doppler (IRD), revealing that TOI-519 b is a planet with a mass of
$0.463^{+0.082}_{-0.088}~M_{\rm Jup}$. We also find that the host star is metal
rich ($\rm [Fe/H] = 0.27 \pm 0.09$ dex) and has the lowest effective
temperature ($T_{\rm eff}=3322 \pm 49$ K) among all stars hosting known
close-in giant planets based on the IRD spectra and mid-resolution infrared
spectra obtained with NASA Infrared Telescope Facility / SpeX. The core mass of
TOI-519 b inferred from a thermal evolution model ranges from $0$ to
$\sim30~M_\oplus$, which can be explained by both the core accretion and disk
instability models as the formation origins of this planet. However, TOI-519 is
in line with the emerging trend that M dwarfs with close-in giant planets tend
to have high metallicity, which may indicate that they formed in the core
accretion model. The system is also consistent with the potential trend that
close-in giant planets around M dwarfs tend to be less massive than those
around FGK dwarfs.
