PROBLEM TO BE SOLVED: To provide a toilet seat apparatus with warm water bidet which has a preferable assembling property without increasing the number of parts as compared with that in a conventional one when a body section including a human body private part washing mechanism inside is assembled to a toilet bowl.SOLUTION: When a body section 20 including a human body private part washing mechanism 21 inside, is assembled to a fixed plate 30 fixed to a toilet bowl 2, a body plate 26 being freely detachable to the fixed plate 30 is provided with a first arm 26b with one side supported by the body plate 26 and the other side displaced upward. On a first free end 71 of the first arm 26b, a first protrusion section 26g is provided which protrudes to the fixed plate 30 side. In the meantime, on the fixed plate 30, a first fixed lock section 35L is provided which includes a first fixed lock tapered surface 37L, causes the first protrusion section 26g to displace vertically by the first fixed lock tapered surface 3
