The toilet seat device includes a function unit arranged in a rear part of the toilet; a toilet seat rotatably attached to the function unit; and a private parts cleaning mechanism part provided in the function unit and having a private parts cleaning function. The private parts cleaning mechanism part has a cleaning water flow passage through which cleaning water flows, the cleaning water flow passage is formed with an air gap that opens the cleaning water flow passage to atmosphere, and the air gap is connectable to a tube member for injecting a descaling agent for removing scale accumulated in the cleaning water flow passage.