To provide a toilet seat device, in which sufficient fitting strength is maintained and the removal work of a case is facilitated.

The toilet seat device has a chassis 7 fixed onto a stool and a case 9 installed to the upper section of the chassis 7 and fast stuck to approximately the whole circumference of the base section of the chassis 7. The case 9 has an elastically deformable tongue-shaped member 23 extended towards the chassis 7 from the inside of the case 9, the chassis 7 has a fitting section 17 fitted to the tongue member 23, the tongue member 23 is elastically deformed, and the tongue member 23 is removed from the fitting section 17.