A seat comprising: a frame for a sitting portion; a sitting surface portion; a base seat whose front end side is fixed to a front frame of the frame for a sitting portion; and a support member disposed at a lower side of the base seat, and elastically supported at the frame for a sitting portion so as to support a femoral region of a seated person, wherein, a width of the base seat is smaller than a distance between a right side frame of the frame for a sitting portion and a left side frame of the frame for a sitting portion so as to form gaps respectively between right and left side edge portions of the base seat and the right and left side frames characterised in that the rear end side of the base seat is connected to a rear frame of the frame for a sitting portion via an elastic member; a left end side of the support member is connected to the left side frame via an elastic member, and a right end side of the support member is connected to the right side frame via an elastic member; a front portion and a rear portion of the left end side of the support member are independently connected to the left side frame via elastic members (50, 52), and a front portion and a rear portion of the right end side of the support member are independently connected to the right side frame via elastic members (50, 52); wherein spring constants of the elastic members which connect the rear portion of the left end side and the rear portion of the right end side of the support member to the left and right side frames corresponding thereto, are larger than spring constants of the elastic members which connect the front portion of the left end side and the front portion of the right end side of the support member to the left and right side frames corresponding thereto. a frame for a sitting portion; a sitting surface portion; a base seat whose front end side is fixed to a front frame of the frame for a sitting portion; and a support member disposed at a lower side of the base seat, and elastically supported at the frame for a sitting portion so as to support a femoral region of a seated person, wherein, a width of the base seat is smaller than a distance between a right side frame of the frame for a sitting portion and a left side frame of the frame for a sitting portion so as to form gaps respectively between right and left side edge portions of the base seat and the right and left side frames characterised in that the rear end side of the base seat is connected to a rear frame of the frame for a sitting portion via an elastic member; a left end side of the support member is connected to the left side frame via an elastic member, and a right end side of the support member is connected to the right side frame via an elastic member; a front portion and a rear portion of the left end side of the support member are independently connected to the left side frame via elastic members (50, 52), and a front portion and a rear portion of the right end side of the support member are independently connected to the right side frame via elastic members (50, 52); wherein spring constants of the elastic members which connect the rear portion of the left end side and the rear portion of the right end side of the support member to the left and right side frames corresponding thereto, are larger than spring constants of the elastic members which connect the front portion of the left end side and the front portion of the right end side of the support member to the left and right side frames corresponding thereto.