We present the first results of Lya luminosity functions (LFs) of Lya emitters (LAEs) at z=5.7 and 6.6 derived by the narrowband imaging survey of Subaru/Hyper Suprime-Cam (HSC) SSP. We use the combination of the narrowband and broadband imaging data taken from our on-going HSC survey that targets four independent blank fields. The early data set of the survey identifies a total of 302 LAEs at z=6.6 (5.7) down to log L(Lya) ~ 42.7 erg/s in the area of 18 deg^2 (6 deg^2) for LAEs that is about an order of magnitude larger than those of the previous LAE observations. We report the progresses of our determinations of Lya LF evolution that is sensitive to neutral hydrogen fraction of the IGM, and show early HSC constraints on the neutral hydrogen fraction of the IGM at z=6.6, together with the latest Planck 2016 Thomson scattering optical depth measurements.