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 β which is formed by the virtual plane (7a) and a rear-side bevel (9a) to an angle α 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 θ which is formed by the virtual plane (7a) and a rear-side bevel to the angle α which is formed with the obverse-side bevel (8).
