Abstract: Provided is a novel device or a novel implantation device. The implantation device is one of the following: (1) an implantation device comprising a material (A) having a density of 12 mg/cm3 or more and a dissolution rate of 80% or less as determined after immersion of the material (A) in physiological saline at 37° C. for 24 hours; and (2) an implantation device comprising a material (A) having a dissolution rate of 80% or less as determined after immersion of the material (A) in physiological saline at 37° C. for 24 hours and a ratio of X/Y of 0.24 or more wherein X is a density (mg/cm3) of the material (A) and Y is a degree of swelling of the material (A) expressed in terms of fold increase as determined after immersion of the material (A) in physiological saline at 37° C. for 60 minutes.