To improve a lifting device for a toilet seat which utilizes tap water as the pressure source, and to make the motion sure and smooth, and at the same tame, to miniaturize the overall device.

This toilet seat-lifting device has a cylinder part which is attached to a toilet body, and a piston part which is freely vertically movably attached to the cylinder part, is lifted by tap water fed to the cylinder part, and supports the toilet seat. The toilet seat-lifting device is equipped with hydraulic actuators which are respectively arranged on the right and left sides of the toilet body, and lift-guiding parts which are respectively arranged on the right and left sides of the toilet body and guide the toilet seat in the lifting direction.