Emission lines in galaxies are probed up to the epoch of reoinization (EoR; z ~ 6–10) by deep optical to radio observations. Specifically, ALMA observations are uncovering physical states of inter-stellar medium (ISM) and circum-galactic medium of galaxies at z≳7, while massive optical surveys reveal statistical properties with very large samples of emission line galaxies at z≲7. Here I introduce the progresses of ALMA observations for cold ISM, and review results from the large surveys for Lyman alpha emitters including the on-going Subaru SILVERRUSH project. This talk also presents the recent progresses of new extremely metal poor (strong-line emitting) galaxy project by Subaru observations and machine-learning technique, coming back to z~0 for witnessing early phase of galaxy formation, that complements the efforts of emission-line galaxy studies at high redshift. I conclude my talk with a theoretical forecast of EoR 21cm emission and Subaru emission-line galaxy cross correlation study for understanding physical processes of cosmic reionization.