A vehicle body reinforcement device comprises an elongated shape with its middle portion comprising a hydraulic damper that produces a damping force against longitudinal deformation. Two vehicle body reinforcement devices may be mounted in a longitudinal manner to a vehicle body frame with a lateral space being defined between the two devices. Both ends of each of the vehicle body reinforcement devices can be removably attached from outside of the vehicle to positions that are exposed outside of the passenger compartment and below the vehicle body.