Recently, the family Polyomaviridae was classified as 3 genera, such as Orthopolyomavirus, Wukipolyomavirus which contain mammalian polyomaviruses and Avipolyomavirus which only contain avian polyomaviruses. We have recently isolated novel polyomaviruses, including Mastomys Polyoamvirus (MasPyV) and Vervet monkey Polyoamvirus-1 (VmPyV-1) by epidemiological activities and examined functions of their encoding proteins. In addition, we have been investigating the mechanisms of replication of human polyomavirus, JC polyomavirus (JCPyV). We recently obtained the results of function of JCVPyV-encoding proteins, including early protein (Large T antigen) and late proteins (VP1 and Agno). In this review, we summarized the data of our basic research activities.