PROBLEM TO BE SOLVED: To solve the problems that a mechanical fixation method for fixing a waterproof sheet by using a disk for fixation is performed in sheet waterproof structure on the roof floor of a building, here, there is a case that a problem that the disk for fixation deforms to warp like a recess when the disk for fixation is strongly tightened with a screw for fixation, a joint part between the disk for fixation and the waterproof sheet becomes small, and joining between the disk for fixation and the waterproof sheet becomes insufficient when the disk for fixation deforms like the recess in this way.SOLUTION: In a disk for fixation, first reinforcement groove parts are provided in the surroundings of a screw hole, and second reinforcement groove parts are further provided in the surroundings of the first reinforcement groove parts. Then, the first reinforcement groove parts are provided by being divided, approximately uniformly arranged in the surroundings of the screw hole, and the second reinforce
