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 ; \sim 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/ .