A method of performing diagnosis or treatment perendoscopically in which a guide catheter is inserted inside a body and the guide catheter is made to protrude beyond a distal end portion of the endoscope and in which a first diagnosis or treatment is performed by inserting a medical treatment device that performs the diagnosis or treatment of the body inside the guide catheter and causing this medical treatment device to protrude by a first predetermined distance from the guide catheter and in which the medical treatment device is extracted from the guide catheter and in which a second diagnosis or treatment is performed by inserting a medical treatment device into the guide catheter and causing the medical treatment device to protrude by a second distance which is adjusted in accordance with the first predetermined distance.
