  GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer)
is a mission candidate for a JAXA's strategic L-class mission to be launched in
the 2030s. Its primary sciences are two-fold: galaxy formation and evolution
and planetary system formation and evolution. The GREX-PLUS spacecraft will
carry a 1.2 m primary mirror aperture telescope cooled down to 50 K. The two
science instruments will be onboard: a wide-field camera in the 2-8 $\mu$m
wavelength band and a high resolution spectrometer with a wavelength resolution
of 30,000 in the 10-18 $\mu$m band. The GREX-PLUS wide-field camera aims to
detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high
resolution spectrometer aims to identify the location of the water ``snow
line'' in proto-planetary disks. Both instruments will provide unique data sets
for a broad range of scientific topics including galaxy mass assembly, origin
of supermassive blackholes, infrared background radiation, molecular
spectroscopy in the interstellar medium, transit spectroscopy for exoplanet
atmosphere, planetary atmosphere in the Solar system, and so on.
