A side airbag device, comprising: an airbag; and an inflator which is provided inside the airbag and which, during a side collision, receives an output signal from a sensor, supplies gas to the airbag in a folded state, and deploys the airbag, wherein the airbag is configured such that a tensioned tether is attached to an outer surface on a side that faces an occupant when deployed, the tensioned tether is shaped such that a vertical length of a front edge thereof which is positioned at a front end of the airbag when deployed is longer than a vertical length of a rear edge thereof which is positioned at a rear end of the airbag when deployed, the front edge is shaped so as to continuously spread in the vertical direction along the front end of the airbag between an upper part of the airbag to be on a ceiling side of a vehicle and a lower part of the airbag to be on a floor side of the vehicle when the airbag is deployed, the rear edge is shaped so as to spread in the vertical direction in the vicinity of the rear end of the airbag between the upper part and the lower part of the airbag when deployed, and the front edge of the tensioned tether is continuously joined in the vertical direction to the vicinity of the front end of the airbag, and the tensioned tether is attached so as to move in a state of being pulled in an oblique direction which is inclined with respect to the vertical direction from the front end to the rear end of the airbag when deployed, wherein the oblique direction is a direction which combines a horizontal direction with respect to a vehicle longitudinal direction with a vertical direction that is perpendicular to the horizontal direction, and is a direction in which a vehicle forward side of the tensioned tether is positioned upward relative to a vehicle rearward side of the tensioned tether. an airbag; and an inflator which is provided inside the airbag and which, during a side collision, receives an output signal from a sensor, supplies gas to the airbag in a folded state, and deploys the airbag, wherein the airbag is configured such that a tensioned tether is attached to an outer surface on a side that faces an occupant when deployed, the tensioned tether is shaped such that a vertical length of a front edge thereof which is positioned at a front end of the airbag when deployed is longer than a vertical length of a rear edge thereof which is positioned at a rear end of the airbag when deployed, the front edge is shaped so as to continuously spread in the vertical direction along the front end of the airbag between an upper part of the airbag to be on a ceiling side of a vehicle and a lower part of the airbag to be on a floor side of the vehicle when the airbag is deployed, the rear edge is shaped so as to spread in the vertical direction in the vicinity of the rear end of the airbag between the upper part and the lower part of the airbag when deployed, and the front edge of the tensioned tether is continuously joined in the vertical direction to the vicinity of the front end of the airbag, and the tensioned tether is attached so as to move in a state of being pulled in an oblique direction which is inclined with respect to the vertical direction from the front end to the rear end of the airbag when deployed,