A length adjustable cannula includes: a pipe-like main body having a through hole formed therein and an outer circumferential surface on which a screw thread is formed; a base grip provided at an end of the main body; an inner support plate provided at another end of the main body; and an outer support plate thread-coupled with the outer circumferential surface of the main body, the outer support plate being movable along the screw thread in a lengthwise direction of the main body. A position of the outer support plate is adjusted based on a distance from an outside skin to a medical treatment region of a human body, such that the outer support plate is brought into contact with the outside skin and the inner support plate is brought into contact with and supported by an inside skin of the human body.