There are 2 ferry routes operating between Sado Island and Japan offering you combined total of 84 sailings per week. Sado Kisen operates 2 routes, Ryotsu to Niigata runs 5 times per day & Ogi to Naoetsu about 14 times weekly.
As the frequency and duration of crossings on some routes varies we would advise that you do a live search for crossings from Sado Island to Japan to get the most up to date information.