Provided is a spacer for ionizing radiation therapy, which can be easily disposed in a living body, does not require abdominal surgery for removing the disposed spacer after radiation therapy, and can effectively separate a tissue to be treated from other tissues. The spacer for ionizing radiation therapy includes a fiber assembly obtained by three-dimensionally entangling a fibrous material formed of a biocompatible synthetic polymeric material. Specifically, the spacer has a thickness of 1 mm to 100 mm.