Kira and Sophia have been best friends since they were five years old. They share an apartment together, and tell each other everything.Everyone loves Sophia, and she is often called the advice girl by people who know her. Kira believes she knows her friend better than anyone. But in a mysterious accident, Sophia loses her life.Kira is mourning the devastating loss when she finds a note addressed to her from beyond the grave. Kira realizes she doesn't know her friend as well as she thought, as she starts to uncover the truth. She falls into a deep depression, confused and lost, until a visit from a strange man puts her in the middle of a war between two secret societies.Kira soon understands that she is in danger, anddescribed above is comprised of a screw shaft part 53 having threads 52 at an outer circumference of a shaft part 51 of a straight column-like shape, a drilling part 54 connected to an extreme end of the screw shaft part 53 and a head part 55 arranged at a rear end part of the screw shaft part 53.
The drilling part 54 is formed with a tapered shaft part 56 formed to have a gradually decreased diameter from the shaft part 51 toward its extreme end and an extreme end thread 57 connected to the thread 52 at an outer circumference of the tapered shaft part 56, wherein the fastening is carried out while the drilling part 54 cooperates with the screw shaft part 53 to perform a