Provided is a medical knife which makes it possible to maintain low penetration resistance, without the maximum-width parts of the medical knife creating sudden changes in penetration resistance even when passing through tissue. A knife comprises: a pointed tip; a cutting part which has edges consisting of first oblique faces which are formed along the outer periphery and second oblique faces which are formed inside the first oblique faces; maximum-width parts; and a shank part which is formed contiguously with the maximum-width parts. The cutting part is formed in such a way that the shape of the planar projection thereof becomes wider from the pointed tip towards the maximum-width parts, and the first oblique faces extend further towards the shank part side than the second oblique faces over at least one portion of the area running from the maximum-width parts to the shank part.