The purpose of the present invention is to provide a knife for use in medical treatment, with which it is possible to form an incision which is strongly self-closing. Provided is a knife (A), in which a planar projection shape of a blade part (1) is formed in an approximately rhomboid shape, which comprises: a leading end part (3) which includes a pointed end (2) which is formed in a leading end portion and the vicinity of the pointed end (2)； a maximum width part (4) which is the widest part； and lateral surface parts (5) which are formed between the leading end part (3) and the maximum width part (4). With a virtual plane (7a) as a boundary, said virtual plane (7a) including edges (7) which are formed along the outer circumference of the blade part (1), an obverse-side bevel (8) which configures the edges (7) is formed on the obverse side and a reverse-side bevel (9) which configures the edges (7) is formed on the reverse side. A ratio in the leading end part (3), of an angle ß which is formed by the virtual plane (7a) and a rear-side bevel (9a) to an angle a which is formed by the virtual plane (7a) and the obverse-side bevel (8), is greater than a ratio in the lateral surface parts (5), of an angle ?¸ which is formed by the virtual plane (7a) and a rear-side bevel to the angle a which is formed with the obverse-side bevel (8).Se proporciona una cuchilla médica capaz de realizar una incisión que tiene una propiedad altamente auto-cerrable. Una cuchilla (A) tiene una forma de proyección plana de una parte de corte (1) formado en una forma romboidal aproximada que incluye una parte extrema frontal (3) que incluye un extremo puntiagudo (2) formado en una porción extrema frontal y la vecindad del extremo puntiagudo (2), una parte más ancha (4) con el mayor ancho, y una parte de superficie lateral (5) formada entre la parte extrema frontal (3) y la parte más ancha (4), un bisel lateral frontal (8) que constituye un borde (7) en el lado frontal y en el bisel lateral po
