A medical knife has a cutting edge formed along the outer edge of a blade portion, and the thickness of the lower portion located under a boundary acting as a surface including the cutting edge is made thinner than that of an upper portion located on the boundary, or a lower surface is composed of the surface including the cutting edge. The surface of the upper portion on the above surface including the cutting edge is composed of a first slanting surface constituting the cutting edge, a second reflecting surface, and a flat surface composed of a non-reflecting surface as well as the lower portion has a lower slanting surface 10 and a lower flat surface 11 each composed of a reflecting surface. With the above arrangement, the medical knife can reduce the amount of reflected light when an operation such as an ophthalmic operation is executed under a microscope.