An airbag system, onto which an airbag can be mounted for restraining an occupant during a side collision of a vehicle, can be used as an armrest. The airbag system has a container having an airbag module inside and a supporting bar for supporting the container, wherein a first end of the supporting bar is pivotally connected to a rear of a seat portion and a second end of the supporting bar is connected to the container. When an impact force is applied to a side of a vehicle, because the supporting bar and the container are transferred from a first position to a second position by first and second step motors, the supporting bar can absorb the impact force transmitted through a door of the vehicle. The airbag module contained in the container is operated and the inflator deploys the airbag, thus the occupant is protected.