A pre-crash side airbag device includes a side collision detection sensor, a pre-crash sensor, a main airbag, an auxiliary airbag, and a control unit. The side collision detection sensor detects a collision occurring at the side of a vehicle. The pre-crash sensor detects physical quantities related to the speed of an approaching object. The main airbag is deployed when a control signal is input. The auxiliary airbag is coupled to one side of the main airbag. The control unit controls the deployment of the main airbag and the auxiliary airbag depending on a pre-crash case where a high-speed side collision is expected and a normal case where a low-speed side collision is expected or the measurement value is erroneous.