A side airbag structure and a seatback of a seat for a vehicle having a side support provided outside of the seatback in a vehicle width direction, wherein the side support is capable of supporting a passenger of the vehicle, the side airbag structure (1 a) comprising: a side airbag unit to be disposed in the side support and to be attached to a seatback frame of the seatback, wherein the side airbag unit includes an airbag that inflates frontward in a vehicle collision or its prediction; a cushion pad covering at least a front side of the side airbag unit; a seat skin enveloping the side airbag unit and the cushion pad; a first fragile part provided on the seat skin, wherein the first fragile part is cleaved by an inflation of the airbag and allows the airbag to deploy through the cleaved first fragile part and out of the seat skin; wherein the seat skin includes a front skin at least partly covering a front side of the cushion pad at least between an inner end of the side support in the vehicle width direction and the first fragile part; and a first slit (24; 24a) provided on the cushion pad in the side support and substantially inside of the first fragile part in the vehicle width direction, wherein the first slit (24; 24a) passes through the cushion pad substantially in a vehicle longitudinal direction; wherein the cushion pad located substantially outside of the first slit (24; 24a) in the vehicle width direction and substantially inside of the first fragile part in the vehicle width direction is moved in a direction substantially apart from the passenger in a top view upon the inflation of the airbag; a first low expansion member is disposed substantially inside of the front skin, wherein one end (41 b) of the first low expansion member is attached to the inside of the front skin in the vicinity of the first fragile part and the other end (41 a) of the first low expansion member is to be attached to the seatback frame through the first slit, so that the first low expansion member can substantially inhibit the front skin from expanding due to the inflation of the airbag; and the seat skin includes a side skin that at least partly covers the cushion pad at least between a side end of the side support in the vehicle width direction and the first fragile part; characterized by a second low expansion member disposed substantially inside of the side skin, wherein one end of the second low expansion member is attached to the inside of the side skin in the vicinity of the first fragile part and the other end of the second low expansion member is to be attached to the seatback frame; and a second slit (25; 25a) provided on the cushion pad and behind the first fragile part in the vehicle longitudinal direction, wherein the second slit (25; 25a) allows the second low expansion member to pass through the cushion pad in the vehicle width direction. a side airbag unit to be disposed in the side support and to be attached to a seatback frame of the seatback, wherein the side airbag unit includes an airbag that inflates frontward in a vehicle collision or its prediction; a cushion pad covering at least a front side of the side airbag unit; a seat skin enveloping the side airbag unit and the cushion pad; a first fragile part provided on the seat skin, wherein the first fragile part is cleaved by an inflation of the airbag and allows the airbag to deploy through the cleaved first fragile part and out of the seat skin; wherein the seat skin includes a front skin at least partly covering a front side of the cushion pad at least between an inner end of the side support in the vehicle width direction and the first fragile part; and a first slit (24; 24a) provided on the cushion pad in the side support and substantially inside of the first fragile part in the vehicle width direction, wherein the first slit (24; 24a) passes through the cushion pad substantially in a vehicle longitudinal direction; wherein the cushion pad located substantially outside of the first slit (24; 24a) in the vehicle width direction and substantially inside of the first fragile part in the vehicle width direction is moved in a direction substantially apart from the passenger in a top view upon the inflation of the airbag; a first low expansion member is disposed substantially inside of the front skin, wherein one end (41 b) of the first low expansion member is attached to the inside of the front skin in the vicinity of the first fragile part and the other end (41 a) of the first low expansion member is to be attached to the seatback frame through the first slit, so that the first low expansion member can substantially inhibit the front skin from expanding due to the inflation of the airbag; and the seat skin includes a side skin that at least partly covers the cushion pad at least between a side end of the side support in the vehicle width direction and the first fragile part; characterized by a second low expansion member disposed substantially inside of the side skin, wherein one end of the second low expansion member is attached to the inside of the side skin in the vicinity of the first fragile part and the other end of the second low expansion member is to be attached to the seatback frame; and a second slit (25; 25a) provided on the cushion pad and behind the first fragile part in the vehicle longitudinal direction, wherein the second slit (25; 25a) allows the second low expansion member to pass through the cushion pad in the vehicle width direction.