A medical treatment device enables an injection needle to accurately puncture brain tissue at a prescribed position and a drug solution to be injected into brain tissue, while avoiding damage to the brain tissue as the injection needle is handled. The use of the medical treatment device involves inflating a first balloon located on the outside of a catheter, directing a distal portion of a puncture needle, which is located inside the catheter, to outside the catheter through an opening in the catheter, after which the first balloon is deflated and a second balloon is inflated, and moving the catheter towards the brain tissue located on the opening side so that the injection needle projecting from the opening punctures the brain region.