A seat which is employed in combination with a seatbelt system for a vehicle designed to protect an occupant when an emergency situation of the vehicle occurs. When the tension acting on the seatbelt exceeds a predetermined value, a movable member connected to a seatbelt anchor member causes a push-up member to move upwards within a seat cushion. As a result, the push-up member forms a protuberance at the front part of the seat cushion, whereby the movement of the occupant's waist toward the front end of the vehicle is limited.