Hyper Suprime-Cam is an instrument on the Subaru telescope with wide field-of-view (1.77deg2) and has the most efficient wide-field deep imaging capability per unit time and thus the most powerful instrument to detect transients. To maximize scientific outcomes from HSC, we developed a quick image subtraction system to realize prompt detection of transients. The system realizes realtime optical transient finding in a transient survey performed in Subaru Strategic Program and in target of opportunity observations following up transients found by other facilities, e.g., a fast radio burst and an optical counterpart of gravitational waves. In this presentation, I summarize results of transient survey and multi-messenger astronomy with Subaru/Hyper Suprime-Cam.