To provide fastening technology with a flat head bolt for restricting the rotation of the flat head bolt in the loosening direction while permitting the rotation thereof in the joining direction and for preventing the associative rotation of a anti-loosening taper ring.

In this fastening structure with the flat head bolt, a fastened object 2 is fastened to a base member in an overlapped condition by inserting a screw portion 10 of the flat head bolt 1 through a counter tapered bore 20 and joining it to the base member 3. A locking protruded portion 40 is formed on the anti-loosing taper ring which is mounted between a tapered face 14 formed on a head 11 of the flat head bolt and the counter tapered bore of the fastened object, and the locking protruded portion 40 engages with a locking groove 21 formed in the counter tapered bore of the fastened object. When the flat head bolt is rotated in the joining direction, a cutout groove 13 formed in the head of the flat head bolt rides over a locking pawl 41 formed at the upper edge of the anti-loosing taper ring, and when it is rotated in the loosening direction, the cutout groove engages with the locking pawl.