In a passenger protection device, an object detection unit detects a relative position of an object in a circumference of a vehicle to the vehicle. A side collision prediction unit predicts that the object is about to collide with a side face of the vehicle, based on the relative position detected by the object detection unit. An impact detection unit detects an impact of the object on the vehicle side face. A side collision judgment unit determines whether the object has collided with the vehicle side face, based on the impact detected by the impact detection unit. A passenger protection unit protects a passenger in the vehicle. When the side collision prediction unit predicts that the object is about to collide with the vehicle side face and the side collision judgment unit determines that the object has collided with the vehicle side face, the passenger protection unit is initiated.