To save the space to provide a filter cassette of a vertically illuminating fluorescent microscope.

In the vertically illuminating fluorescent microscope provided with a turret type filter cassette capable of switching of plural exciting/ absorbing filters 20 and 28 and dichroic mirrors 22, exciting/absorbing filters and dichroic mirrors are directly attached to a turret 106 which has a shape obtained by cutting away the acute part of an octagonal pyramid having the axis of revolution as the axis by a face orthogonal to the axis and has each pyramidal face inclined at 45° to the optical axis of illumination and that of observation. A through hole to secure an optical path is provided in each pyramid face, and the trapezoidal dichroic mirror 22 is attached so as to close this hole. The lower end of the dichroic mirror 22 is pressed to the pyramidal face by a spring 23 so that the upper end of the dichroic mirror may butt against a cylindrical exciting filter frame.