A side impact airbag apparatus comprising: an airbag, which is fixed to a vehicle via an attaching portion and is configured to be deployed and inflated diagonally forward and upward in the vicinity of a side of an occupant seated in a vehicle seat and from a part at which the airbag is fixed to the vehicle via the attaching portion, wherein the airbag includes a front end, and the attaching portion is located in a rear lower portion of the airbag; a gas generator, which supplies inflation gas to the airbag in response to an impact applied to the vehicle from a side of the vehicle seat; and a strap having a front end attached to the front end of the airbag and a rear end attached to one of the part of the airbag at which the airbag is fixed to the vehicle and the vicinity of that fixed part, wherein the airbag includes a first section, and the strap includes an upper strap portion, which is tensioned when the airbag is deployed and inflated, and a lower strap portion, which is located below the upper strap portion and is tensioned with a tensile force greater than that of the upper strap portion when the airbag is deployed and inflated, characterized in that the front end of the airbag includes an upper front attachment portion and a lower front attachment portion, the rear lower portion of the airbag includes an upper rear attachment portion and a lower rear attachment portion, a front end of the upper strap portion is attached to the upper front attachment portion of the airbag, a rear end of the upper strap portion is attached to the upper rear attachment portion of the airbag, a front end of the lower strap portion is attached to the lower front attachment portion of the airbag, a rear end of the lower strap portion is attached to the lower rear attachment portion of the airbag, a state of the airbag in which the airbag is deployed in a planar form without being filled with inflation gas is defined as an uninflated and deployed state, a distance between the front end and the rear end of the upper strap portion in a deployed state is defined as a first distance, a distance between the upper front attachment portion and the upper rear attachment portion of the airbag in the uninflated and deployed state is defined as a second distance, the first distance is set to be shorter than the second distance, a distance between the front end and the rear end of the lower strap portion in a deployed state is defined as a third distance, a distance between the lower front attachment portion and the lower rear attachment portion of the airbag in the uninflated and deployed state is defined as a fourth distance, the third distance is set to be shorter than the fourth distance, and a difference between the fourth distance and the third distance is set to be greater than a difference between the second distance and the first distance. an airbag, which is fixed to a vehicle via an attaching portion and is configured to be deployed and inflated diagonally forward and upward in the vicinity of a side of an occupant seated in a vehicle seat and from a part at which the airbag is fixed to the vehicle via the attaching portion, wherein the airbag includes a front end, and the attaching portion is located in a rear lower portion of the airbag; a gas generator, which supplies inflation gas to the airbag in response to an impact applied to the vehicle from a side of the vehicle seat; and a strap having a front end attached to the front end of the airbag and a rear end attached to one of the part of the airbag at which the airbag is fixed to the vehicle and the vicinity of that fixed part, wherein the airbag includes a first section, and the strap includes an upper strap portion, which is tensioned when the airbag is deployed and inflated, and a lower strap portion, which is located below the upper strap portion and is tensioned with a tensile force greater than that of the upper strap portion when the airbag is deployed and inflated, characterized in that the front end of the airbag includes an upper front attachment portion and a lower front attachment portion, the rear lower portion of the airbag includes an upper rear attachment portion and a lower rear attachment portion, a front end of the upper strap portion is attached to the upper front attachment portion of the airbag, a rear end of the upper strap portion is attached to the upper rear attachment portion of the airbag, a front end of the lower strap portion is attached to the lower front attachment portion of the airbag, a rear end of the lower strap portion is attached to the lower rear attachment portion of the airbag, a state of the airbag in which the airbag is deployed in a planar form without being filled with inflation gas is defined as an uninflated and deployed state, a distance between the front end and the rear end of the upper strap portion in a deployed state is defined as a first distance, a distance between the upper front attachment portion and the upper rear attachment portion of the airbag in the uninflated and deployed state is defined as a second distance, the first distance is set to be shorter than the second distance, a distance between the front end and the rear end of the lower strap portion in a deployed state is defined as a third distance, a distance between the lower front attachment portion and the lower rear attachment portion of the airbag in the uninflated and deployed state is defined as a fourth distance, the third distance is set to be shorter than the fourth distance, and a difference between the fourth distance and the third distance is set to be greater than a difference between the second distance and the first distance.