A vehicular seat comprising: a side airbag device that inflates and deploys a side airbag with which a side airbag module that is provided in a side support portion of a seat back is equipped, forward of the side support portion with respect to the seat; a side support adjustment mechanism that has an air bag that is provided in the side support portion inside the side airbag module in a seat width direction, and that inflates and contracts the airbag: and a support body (49; 60; 76) that is provided between the side airbag module and the air bag, that is supported by a skeleton of the seat back, that supports the air bag from outside in the seat width direction at a time of inflation of the air bag, and that is displaced inward in the seat width direction together with the air bag due to an inflation pressure of the side airbag airbag at a time of inflation and deployment of the side airbag characterized in that the support body (49; 60) has a fixed portion (52; 62) that is fixed to the skeleton of the seat back, and a movable portion (56; 66) that is coupled at an end portion thereof on a rear side with respect to the seat to the fixed portion via a hinge and that is arranged between the side airbag module and the air bag. a side airbag device that inflates and deploys a side airbag with which a side airbag module that is provided in a side support portion of a seat back is equipped, forward of the side support portion with respect to the seat; a side support adjustment mechanism that has an air bag that is provided in the side support portion inside the side airbag module in a seat width direction, and that inflates and contracts the airbag: and a support body (49; 60; 76) that is provided between the side airbag module and the air bag, that is supported by a skeleton of the seat back, that supports the air bag from outside in the seat width direction at a time of inflation of the air bag, and that is displaced inward in the seat width direction together with the air bag due to an inflation pressure of the side airbag airbag at a time of inflation and deployment of the side airbag characterized in that the support body (49; 60) has a fixed portion (52; 62) that is fixed to the skeleton of the seat back, and a movable portion (56; 66) that is coupled at an end portion thereof on a rear side with respect to the seat to the fixed portion via a hinge and that is arranged between the side airbag module and the air bag. a support body (49; 60; 76) that is provided between the side airbag module and the air bag, that is supported by a skeleton of the seat back, that supports the air bag from outside in the seat width direction at a time of inflation of the air bag, and that is displaced inward in the seat width direction together with the air bag due to an inflation pressure of the side airbag airbag at a time of inflation and deployment of the side airbag characterized in that the support body (49; 60) has a fixed portion (52; 62) that is fixed to the skeleton of the seat back, and a movable portion (56; 66) that is coupled at an end portion thereof on a rear side with respect to the seat to the fixed portion via a hinge and that is arranged between the side airbag module and the air bag.