To remove an existing stool lid and assemble a toilet seat for child simply from above by utilizing a rotary shaft of this stool lid by attaching and detaching the toilet seat for child freely on the rotary shaft supporting a toilet seat attached to a device body arranged in a stool so as to rotate.

When a toilet seat 3 for child is removed, a knob 16 is turned in Y-direction to release the engagment of a claw, and a force is applied continuously in Y-direction so that a C-ring part 14 of a member 13 for attaching and detaching slides in a guide groove. An opening 12 and a shaft hole 8b are communicated due to the rotation of a shut-off part 15, and a rotary shaft 2b is pulled out along this communication passage to remove the toilet seat 3 for child from a toilet seat 4. When the toilet seat 3 for child is mounted, the rotary shaft 2b is inserted into the shaft hole 8b through the opening 12 on the contrary, and a member 14 for attaching and detaching is turned in X-direction. Consequently, the shaft hole 8b and the opening 12 are shut off in a base part 14 to prevent coming off of the rotary shaft 2b so that the toilet seat 3 for child is mounted on the toilet seat 4.