A medical knife includes a blade plate and a blade portion formed at the distal end of the blade plate. Cutting edges extend from the distal end to the proximal end of the blade portion. Two blade surfaces in the width direction of a back-face portion of the blade portion are formed to gradually increase a distance between opposite side and cutting edge from one of the distal end and the proximal end to the other. The two blade surfaces in the width direction of a front-face portion of the blade portion are formed such that the distance between the opposite side and the cutting edge becomes equal in a zone corresponding to a distal end of the cutting edge and a zone corresponding to a proximal end of the cutting edge, over the entire cutting edge from the distal end to the proximal end of the cutting edge.