PURPOSE: To obtain a clear static image having no shade, by providing a prism which is disposed in alignment of optical axis to the axial center of rotation of a rotating body to be observed, and rotates at the speed synchronized with the rotating speed of said body.

CONSTITUTION: An observation device for a high speed rotating body has a synchronous motor 1, a rotating body 2 and a semitransparent prism as a reflective transmission means of which the optical axis is aligned to the axial center of rotation of the body 2. Part of light from a light source 5 is reflected with a semitransparent prism 4 so that the light is projected to the body 2 along the axial center of rotation. If such constitution is used, the light from the light source is projected to the rotating body along the axial center of rotation by a reflective transmission means such as a semitransparent prism and the light from the rotating body is made incident to a rotary prism, so that the static image of the rotating body is obtd. by the exit light from said prism. The clear image having no shade is thus obtd.