PROBLEM TO BE SOLVED: To provide an insulating cover which is easy to attach before surgery and easy to remove after surgery, and is not likely to come off during surgery, and to provide a surgical treatment instrument including it.SOLUTION: In a manipulator 1 (a surgical treatment instrument), the insulating cover 10 is mounted on the distal end of a shaft 18 of a manipulator body 11. The insulating cover 10 includes: a cover body 102 and a cylindrical collar member 104 surrounding the base end of the cover body 102. The cover body 102 has a first tapered part 102a gradually reduced in outer diameter as it goes toward the base end and a base end constituting part 114 inserted in the collar member 104 on the side closer to the base end than the first tapered part 102a. The collar member 104 has a second tapered part 104a gradually reduced in inner diameter as it goes toward the base end. When the collar member 104 is moved toward the distal end, the first tapered part 102a is pressed by the first tapered part 104a so that the first tapered part 102a is pressed inward.
