Hydrogen- and helium-ionizing photons from stars and quasars can propagate through the intergalactic medium (IGM)
and change the chemical and kinematical properties of IGM through ionization and heating.
This process, the cosmic reionization, is believed to have started with the birth of first stars and ended
when all the gas in the IGM got ionized due to the plethora of ionizing photons.
Continuous percolation of individual H II regions generate H II regions in cosmological scales,
and this enables us to probe the process itself and the collective properties of radiation sources
through observations targeting large-scale phenomena.
We review our own reionization models that include first stars at high redshift, z~30-15,
and show the current constraint on such models made by the Planck (cosmic microwave background: CMB)
and EDGES (Experiment to Detect the Global EoR Signature, on 21-cm).
We also present how improved CMB and 21-cm observations, e.g. LiteBIRD (Lite (Light) satellite for the studies of B-mode polarization
and Inflation from cosmic background Radiation Detection) and SKA (Square Kilometre Array), can further tighten the constraint
on reionization models.