A medical instrument (1, 1A, 1B) comprising: an insertion part configured to be inserted into a human body; and a driving mechanism coupled to the insertion part, wherein the insertion part has: a treatment part configured to perform treatment on a treatment target portion; a joint part configured to support the treatment part and to be capable of changing a direction of the treatment part; and a driving force transmission part connected to the joint part and configured to transmit a driving force that changes the direction of the treatment part to the joint part, the driving mechanism has a driving force generator that is connected to the driving force transmission part and is configured to generate the driving force, and a storage part (19, 47) configured to store the treatment part in the joint part by the driving force transmitted from the driving force generator to the joint part via the driving force transmission part is formed at the joint part, characterized in that the joint part has a flexible spring part (14, 42) that is bendably configured to change the direction of the treatment part and has a substantially cylindrical shape; the driving force transmission part has a plurality of linear members (24, 25) that are provided corresponding to a bending direction in bending movement of the joint part and are fixed to a distal end of the flexible spring part (14, 42); the flexible spring part (14, 42) is bent by at least one of the plurality of linear members (24, 25) being pulled; and at least two of the plurality of linear members (24, 25) are uniformly pulled or pressed such that the distal end of the flexible spring part (14, 42) advances or retracts in a center axis direction of the flexible spring part (14, 42), and thereby the storage part (19, 47) is formed adjacent to the distal end of the flexible spring part (14, 42). an insertion part configured to be inserted into a human body; and a driving mechanism coupled to the insertion part, wherein the insertion part has: a treatment part configured to perform treatment on a treatment target portion; a joint part configured to support the treatment part and to be capable of changing a direction of the treatment part; and a driving force transmission part connected to the joint part and configured to transmit a driving force that changes the direction of the treatment part to the joint part, a treatment part configured to perform treatment on a treatment target portion; a joint part configured to support the treatment part and to be capable of changing a direction of the treatment part; and a driving force transmission part connected to the joint part and configured to transmit a driving force that changes the direction of the treatment part to the joint part, the driving mechanism has a driving force generator that is connected to the driving force transmission part and is configured to generate the driving force, and a storage part (19, 47) configured to store the treatment part in the joint part by the driving force transmitted from the driving force generator to the joint part via the driving force transmission part is formed at the joint part, characterized in that the joint part has a flexible spring part (14, 42) that is bendably configured to change the direction of the treatment part and has a substantially cylindrical shape; the driving force transmission part has a plurality of linear members (24, 25) that are provided corresponding to a bending direction in bending movement of the joint part and are fixed to a distal end of the flexible spring part (14, 42); the flexible spring part (14, 42) is bent by at least one of the plurality of linear members (24, 25) being pulled; and at least two of the plurality of linear members (24, 25) are uniformly pulled or pressed such that the distal end of the flexible spring part (14, 42) advances or retracts in a center axis direction of the flexible spring part (14, 42), and thereby the storage part (19, 47) is formed adjacent to the distal end of the flexible spring part (14, 42).