A medical knife, comprising: a sharp tip; a cutting part including a first oblique face formed along an outer periphery, an edge formed of the first oblique face, and a second oblique face formed inside the first oblique face; a maximum-width part formed continuously from the cutting part; and a shank part formed continuously from the maximum-width part, wherein a planar projection shape of the cutting part is formed such that a width dimension increases from a side of the sharp tip toward the maximum-width part, wherein the first oblique face is formed on an outer peripheral side of the second oblique face on a portion from the cutting part through the maximum-width part to the shank, in the planar shape in the maximum-width part, the first oblique face to form the edge obliquely intersects with the maximum-width part of the medical knife and the first oblique face is formed along an outer periphery of the maximum-width part to the shank part side, wherein the cutting part includes a lower oblique face formed on a lower side of the first oblique face, and the edge is formed of the first oblique face formed on an upper side and the lower oblique face formed on the lower side intersecting with each other, and wherein a substantially triangular wedge-shaped surface having an intersection with the edge as an apex and is defined by an end edge of the first oblique face and an end edge of the lower oblique face of which area increases from the edge side toward the shank part side is formed on a side surface of the medical knife characterized in that the first oblique face is formed such that the first oblique face lies on the second oblique face at least at a part of the portion from the maximum-width part to the shank part side, and in that the first oblique face extends further toward a side of the shank part than the second oblique face on at least a part of a portion from the maximum-width part to the shank part side. a sharp tip; a cutting part including a first oblique face formed along an outer periphery, an edge formed of the first oblique face, and a second oblique face formed inside the first oblique face; a maximum-width part formed continuously from the cutting part; and a shank part formed continuously from the maximum-width part, wherein a planar projection shape of the cutting part is formed such that a width dimension increases from a side of the sharp tip toward the maximum-width part, wherein the first oblique face is formed on an outer peripheral side of the second oblique face on a portion from the cutting part through the maximum-width part to the shank, in the planar shape in the maximum-width part, the first oblique face to form the edge obliquely intersects with the maximum-width part of the medical knife and the first oblique face is formed along an outer periphery of the maximum-width part to the shank part side, wherein the cutting part includes a lower oblique face formed on a lower side of the first oblique face, and the edge is formed of the first oblique face formed on an upper side and the lower oblique face formed on the lower side intersecting with each other, and wherein a substantially triangular wedge-shaped surface having an intersection with the edge as an apex and is defined by an end edge of the first oblique face and an end edge of the lower oblique face of which area increases from the edge side toward the shank part side is formed on a side surface of the medical knife characterized in that the first oblique face is formed such that the first oblique face lies on the second oblique face at least at a part of the portion from the maximum-width part to the shank part side, and in that the first oblique face extends further toward a side of the shank part than the second oblique face on at least a part of a portion from the maximum-width part to the shank part side.