  We present the deepest optical images of the COSMOS field based on a joint
dataset taken with Hyper Suprime-Cam (HSC) by the HSC Subaru Strategic Program
(SSP) team and the University of Hawaii (UH). The COSMOS field is one of the
key extragalactic fields with a wealth of deep, multi-wavelength data. However,
the current optical data are not sufficiently deep to match with, e.g., the
UltraVista data in the near-infrared. The SSP team and UH have joined forces to
produce very deep optical images of the COSMOS field by combining data from
both teams. The coadd images reach depths of g=27.8, r=27.7, i=27.6, z=26.8,
and y=26.2 mag at 5 sigma for point sources based on flux uncertainties quoted
by the pipeline and they cover essentially the entire COSMOS 2 square degree
field. The seeing is between 0.6 and 0.9 arcsec on the coadds. We perform
several quality checks and confirm that the data are of science quality; ~2%
photometry and 30 mas astrometry. This accuracy is identical to the Public Data
Release 1 from HSC-SSP. We make the joint dataset including fully calibrated
catalogs of detected objects available to the community at
https://hsc-release.mtk.nao.ac.jp/.
