To prevent the damage to a concrete structure at the time of demolding, to easily correspond to the alteration of the size of an insert instrument, to certainly fix the insert instrument to a frame and to improve workability of demolding work.

A columnar holder 7 is fitted in the sleeve 2 of the insert instrument 6 and a pair of slits 7a and 7b are provided at the holder 7 in an axial direction. The fixing belt 8 inserted in the attaching hole 4a of the frame 4 at one end thereof from the outside of the frame 4 is inserted in the slits 7a and 7b of the holder 7 to be again inserted in the attaching hole 4a and both end parts of the fixing belt 8 are locked with a fixing tool main body 9 while pulled on the outer side of the frame 4 and the holder 7 is firmly fixed to the inside of the frame 4.