To provide a connection device for an optical instrument and the optical instrument which suppress the expansion and deformation of a microscope body due to thermal conduction from a light source to prevent the focus deviation.

An adaptor 40 is a connection device for an optical instrument for connecting a microscope body 1 to a light source 20 and includes an adaptor body 41 for connecting the microscope body 1 to the light source 20, a cooling element 42 disposed outside of the adaptor body 41 with the cooling surface being in contact therewith, a heat radiation member 43 disposed on the heat radiation surface of the cooling element 42, and a control unit 51 for controlling the output of the cooling element 42.