A medicine injector including: a pressurization rod which performs relative movements with respect to a medicine protective cap along an axial line stretching from the medicine protective cap toward one direction to pressurize a liquid medicine contained in the medicine protective cap so as to discharge the liquid medicine from the medicine protective cap and includes a plurality of protrusions which are spirally arranged on an outer surface thereof； a rotary member which includes a guide part for guiding movements of the protrusions and is rotatably and immovably combined with an inner surface of a main case to move the pressurization rod so as to pressurize the liquid medicine, wherein the guide part is formed on an inner surface of the rotary member； a rotating means which is disposed between the rotary member and the main case to rotate the rotary member； and a rotation prevention member which is installed at the main case to contact an outer surface of the rotary member and be released from contacting the outer surface of the rotary member so as to selectively rotate the rotary member.
