To enhance product quality and reliability by enabling the sure purge of factors in the occurrence of backlash in a state in which an accessory unit is mounted.

A fastener 4 for a toilet stool is equipped with: a rod-shaped member 10 which is protrusively provided in the accessary unit 2 and in which an engaging part 13 is provided on an outer periphery of a shaft; a cylindrical body 20 which has an opening 28 for exposing the engaging part 13 of the member 10 which is fixed to a mounting hole 1a of a toilet stool body 1 so as to be inserted inside; and a locking member 40 having an elastic piece 42, which is assembled to the cylindrical body 20 and arranged in the axial direction of a cylinder, and a locking part 43 which is provided in the elastic body 42 and engaged with/disengaged from the engaging part 13 via the opening 28. The cylindrical member 20 is split into: an upper socket 21 in which an external thread 24 is provided on an outer periphery and which is fixed to the body 1; and a lower socket 25 which has the opening 28 and an internal thread 8a located on an upper inner periphery. The lower socket 25 is turnably assembled to the upper socket 21 via the screwing of the external thread 24 into the internal thread 8a; and the locking member 40 and the member 10 can be moved downward by virtue of turning via the external and internal threads 24 and 8a in a state in which the locking part 43 and the engaging part 13 are engaged with each other.