To provide a toilet seat device suppressing the adherence of dirt in a gap between a base cover and a base plate constituting a body part of the toilet seat device.

A recessed part 12 present in the front side of a base plate lower surface 11 of a base plate 10, and a flat surface part 23 of a base cover 20 are overlapped, and a surface 31 having a joint portion 30 between the surface 11 (a lower end part of the recessed part 12) and the flat surface part 23 is fixed such that the surface 31 becomes flush.