The present invention provides a collision detecting device and a collision detecting method, capable of accurately detecting a collision on the basis of a change in the pressure in a pressure chamber. A collision body discriminating system (10) is provided with: a chamber member (18) elongated in the vehicle width direction, having a pressure chamber (24) therein, and provided in front of a bumper reinforcement (14); a absorber (20) elongated in the vehicle width direction and provided in front of the bumper reinforcement (14); a pressure sensor (22) for outputting a signal corresponding to a change in the pressure in the pressure chamber (24); and an ECU (26) for discriminating a collision body on the basis of a signal from the pressure sensor (22). The chamber member (18) and the absorber (20) are arranged in a top-bottom relationship with a gap (C) therebetween, and this allows the chamber member (18) and the absorber (20) to deform independently of each other when the collision body collides with the vehicle.