A vehicular seat slide device, comprising: a first rail which is fixed to one of a vehicular floor and a seat; a second rail which is fixed to the other one of the floor and the seat, and is linked to be movable relative to the first rail; a screw rod which is supported by the second rail to be rotatable around an axial line which extends in a moving direction; a metal case which is fixed to the first rail, and through which the screw rod is inserted; a resin nut (20, 40) which is housed in the metal case in a state of being unmovable in the moving direction with respect to the metal case, and is screwed to the screw rod; and a metal nut (25, 45) which is housed in the metal case in a state of not being in contact with the resin nut (20, 40) in the moving direction, and is screwed to the screw rod; the vehicular seat slide device being characterized in that the metal case includes one pair of attaching portions which are fixed at a certain interval in the moving direction to a wall portion of the first rail which faces the second rail in a vehicle height direction, one pair of erected portions which respectively extend in the vehicle height direction to be close to the second rail from tip ends that are close to each other in the moving direction of both attaching portions, and a connecting portion which connects the tip ends of both erected portions to each other in the moving direction, the screw rod is inserted into both erected portions, and in the vehicle height direction, a wall portion side end of the metal nut (25, 45) which approaches the wall portion is closer to the wall portion than the tip end surface which faces the second rail of the attaching portion. a first rail which is fixed to one of a vehicular floor and a seat; a second rail which is fixed to the other one of the floor and the seat, and is linked to be movable relative to the first rail; a screw rod which is supported by the second rail to be rotatable around an axial line which extends in a moving direction; a metal case which is fixed to the first rail, and through which the screw rod is inserted; a resin nut (20, 40) which is housed in the metal case in a state of being unmovable in the moving direction with respect to the metal case, and is screwed to the screw rod; and a metal nut (25, 45) which is housed in the metal case in a state of not being in contact with the resin nut (20, 40) in the moving direction, and is screwed to the screw rod; the vehicular seat slide device being characterized in that the metal case includes one pair of attaching portions which are fixed at a certain interval in the moving direction to a wall portion of the first rail which faces the second rail in a vehicle height direction, one pair of erected portions which respectively extend in the vehicle height direction to be close to the second rail from tip ends that are close to each other in the moving direction of both attaching portions, and a connecting portion which connects the tip ends of both erected portions to each other in the moving direction, the screw rod is inserted into both erected portions, and in the vehicle height direction, a wall portion side end of the metal nut (25, 45) which approaches the wall portion is closer to the wall portion than the tip end surface which faces the second rail of the attaching portion.