A seat cushion device capable of adjusting hardness of a seat cushion including a seating surface, on which a user is seated, includes a recessed part that is formed in a back surface of the seating surface in the seat cushion, a left plate that faces a back surface of a left thigh holding part of the seating surface, on which a left leg thigh of the user is located, and that is movable inside the recessed part vertically, a right plate that faces a back surface of a right thigh holding part of the seating surface, on which a right leg thigh of the user is located, and that is movable inside the recessed part vertically, and a gap adjustment mechanism that moves the left plate and the right plate vertically in an independent manner, and that can adjust a clearance from the back surface of the seating surface.