Abstract: An optical block which is light in weight and low in price, and has optical path distances equivalent to those obtained with glass has an outside form composed by arranging a polyhedron 8a having a surface of incidence for a blue ray B, a polyhedron 8b having a surface of incidence for a red ray R and a polyhedron 8c having a surface of incidence for a green ray G which are configured as transparent plastic members, and optical members 9a and 9b configured as dichroic filters formed as glass members. The polyhedrons 8a to 8c are coupled by way of the optical members 9a and 9b so as to compose an optical block as a monochromator prism block.