Quasars are one of the brightest UV sources in the universe. It is expected that they locally make large ionizing bubbles during reionization; however the global contribution of quasars to ionizing photons has been a matter of debate due to poor constraints on the z~6 quasar luminosity function (QLF) at the faint end. I will show our current studies on low-luminosity quasars at z>6 based on optical wide-field surveys of the Subaru telescope. Taking advantage of its 8.2m primary mirror and its capability of prime focus instruments, we have succeeded in identifying more than 30 quasars down to zAB~24 mag with our Suprime-Cam and much larger Hyper Suprime-Cam surveys. The derived QLFs suggest that quasars produce only a minor fraction of the ionizing photons required for reionization at z~6. NIR spectra of five HSC quasars were taken in Gemini/GNIRS and VLT/XSHOOTER, from which we find a variety in their BH mass (10^7-9 Msun) and Eddington ratio (0.1-1). The sub-Eddington accreting BHs are likely in the post-active phase of their early growth. Our Cycle-4 ALMA observations show that the star-forming activity of the HSC quasar hosts are lower than that of the most luminous quasars showing starburst-like properties.