To obtain a space which enables an airbag to be rapidly securely developed between an occupant seated on a seat and a side part of a vehicle.

A control computer calculates a distance to an object in a lateral direction of a vehicle and a relative speed between the object in a lateral direction of the vehicle and the vehicle based on received information received from a side collision predictor, and judges possibility of occurrence of the side collision based on the calculation result. When there is a possibility of the side collision, the control computer rotates an electric motor 17 in a normal direction or a reverse direction. Accordingly, a first supporting shaft 22 or a second supporting shaft 23 rotates, and an arm rest 14 moves from a use position shown by a solid line in Fig.4(a) to a first retreated position or a second retreated position which is shown by a dotted line. The arm rest 14 in the retreated position is located outside a developed area of the airbag.

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