To commonly use components when changing the upper and lower mounting points vis-a-vis the car body frame of the front underrun protector which prevents a passenger car from going under a truck in a head-on collision between a passenger car and a truck.

It is so constituted that a bracket 2 of a undrrun protector 1 is connected to a bracket 4 on the front side of the car body, a connecting bar 4 is fit into two circular holes 2A, 4A and connected to them, so that the axial directions of the circular hole 2A and a circular hole 4A are coincided and, their spline grooves 2a, 4a and spline teeth 2b, 4b are coincided in position, and in a state in which the bracket 2 abuts the bracket 4, the connecting bar 5 is fit into the two circular holes 2A, 4A so as to let spline teeth 5b and a spline groove 5a of the connecting bar 5 fit into the spline grooves 2a, 4a and the spline teeth 2b, 4b respectively. Then, after the positional relation between the circular holes 2A and 4A is varied, the upper and lower mounting points vis-s-vis the car body are adjusted through connection by means of the connecting bar 5.