A collision detecting device includes a chamber member providing a chamber, a pressure sensor connected to the chamber member, and a bumper absorber absorbing an impact generated by collision of an object with the vehicle. The bumper absorber has a region, which has a thickness in a front-rear direction of the vehicle. The thickness of the region is equal to or smaller than a predetermined thickness. The region is in contact with at least a portion of the chamber member. The collision is detected based on the pressure of the chamber detected by the pressure sensor. Hereby, a collision detecting device in which the bumper absorber has sufficient rigidity, and the chamber has sufficient deformability in case of collision can be obtained.