A safety device for a vehicle, the safety device comprising: a main armrest; an auxiliary armrest that obliquely extends from a front end portion of the main armrest toward a center of a seat including a seat back; a lateral-body chamber that is mounted in the main armrest and that, when triggered to expand, expands upward along a lateral body of a vehicle occupant and thus laterally holds the lateral body of the vehicle occupant in place; and a front-body abdominal chamber that is mounted in the auxiliary armrest and that, when triggered to expand, expands toward an abdomen of the vehicle occupant and thus holds a front body of the vehicle occupant in place; characterized in that the auxiliary armrest is coupled, with a rotation shaft, to the front end portion of the main armrest, and in that, when an airbag expands, the auxiliary armrest rotates from the front end portion of the main armrest toward the center of the seat, wherein the lateral-body chamber is configured to include: a first airbag chamber that is mounted in the main armrest and that, when triggered to expand, expands upward along the lateral body of the vehicle occupant and thus laterally holds an arm of the vehicle occupant in place, and a second airbag chamber that extends protrusively from an upper end portion of the first airbag chamber toward a neck of the vehicle occupant and thus holds a shoulder of the vehicle occupant in place. a main armrest; an auxiliary armrest that obliquely extends from a front end portion of the main armrest toward a center of a seat including a seat back; a lateral-body chamber that is mounted in the main armrest and that, when triggered to expand, expands upward along a lateral body of a vehicle occupant and thus laterally holds the lateral body of the vehicle occupant in place; and a front-body abdominal chamber that is mounted in the auxiliary armrest and that, when triggered to expand, expands toward an abdomen of the vehicle occupant and thus holds a front body of the vehicle occupant in place; characterized in that the auxiliary armrest is coupled, with a rotation shaft, to the front end portion of the main armrest, and in that, when an airbag expands, the auxiliary armrest rotates from the front end portion of the main armrest toward the center of the seat, wherein the lateral-body chamber is configured to include: a first airbag chamber that is mounted in the main armrest and that, when triggered to expand, expands upward along the lateral body of the vehicle occupant and thus laterally holds an arm of the vehicle occupant in place, and a second airbag chamber that extends protrusively from an upper end portion of the first airbag chamber toward a neck of the vehicle occupant and thus holds a shoulder of the vehicle occupant in place. a first airbag chamber that is mounted in the main armrest and that, when triggered to expand, expands upward along the lateral body of the vehicle occupant and thus laterally holds an arm of the vehicle occupant in place, and a second airbag chamber that extends protrusively from an upper end portion of the first airbag chamber toward a neck of the vehicle occupant and thus holds a shoulder of the vehicle occupant in place.