Vehicle-mounted seat device

A vehicle-mounted seat device, comprising;

• a seatback main frame standing in a vehicle cabin;
• a backrest on which a back of a seated person is to lean;
• a relay disk secured at a center in a width direction of the backrest;
• a plurality of suspension tools spreading radially from the relay disk as a center, the suspension tools being for suspending the backrest such that the backrest is able to swing relative to the seatback main frame with the relay disk as a center;
• a seatback subframe mounted on the backrest, the seatback subframe being capable of swinging together with the backrest relative to the seatback main frame;
• a headrest provided above the backrest, the headrest for supporting a head of the seated person;
• a headrest stay connected to the headrest and extending downward from the headrest;
• a rotation shaft extending in a right-left direction, the rotation shaft for supporting a middle portion of the headrest stay such that the headrest stay is able to rotate relative to the seatback main frame;
• a pressure-receiving body secured on a lower portion of the headrest stay and disposed behind and opposed to the relay disk;
• and
• a provisional connection mechanism for connecting the headrest stay to the seatback main frame to regulate rotation of the headrest stay and for releasing connection between the headrest stay and the seatback main frame upon input of a force greater than a predetermined force in a direction in which the headrest stay leans forward.