(Proposal ID) S16B-001 (PI) Inoue, Akio (Proposal Title) Cosmic HydrOgen Reionization Unveiled with Subaru (CHORUS) (Abstract) We propose legacy deep Hyper Suprime-Cam (HSC) imaging with 5 new custom narrowband (NB) filters, NB387, NB527, NB718, IB945, and NB973 atop the footprints of the HSC/Subaru Strategic Program (SSP) Ultra-Deep survey, COSMOS and SXDS. Combining these imaging data and the SSP's 5 broadbands+NB921, we will address 3 key questions of the cosmic reionization that are actively discussed in theory and observations today: ionization history, source, and topology. We will make a census of Lyman continuum emitting objects, the faintest AGNs, and Population III star-formation at z~2-5, and reveal the evolution of the cosmic Lyman continuum emissivity and its dominant sources. We will also visualize the ionization topology at z~7 with a direct comparison of LBG and LAE distributions, for the first time, and obtain a concordance ionization history in conjunction with the Lyalpha LF and clustering results from the SSP. We have carefully designed these NB observations since 2007, and a total of 100 million yen have been awarded from JSPS for the NB developments. Because our NB data will have legacy value not only for reionization but also a range of ancillary science, we will open the fully-reduced NB data to the community in pace with the SSP data releases.