PURPOSE: To certainly detect the interval between a roof panel and the obstacle thereabove even when a car body is inclined to what extent and to certainly prevent the contact of a vehicle with the obstacle.

CONSTITUTION: The change of the interval between a front axle 16 and a chasis frame 11 is detected by a front vehicle height sensor 23 and the change of the interval between a rear axle 18 and the chasis frame 11 is detected by a rear vehicle height sensor 24. The interval between the front end of the roof panel 14a determining the max. height of a vehicle 10 and the obstacle 28b above the roof panel is detected by a front interval sensor 26 and the interval between the rear end of the roof panel 14a and the obstacle 28b is detected by a rear interval sensor 27. A controller 31 calculates the height difference between the front and rear ends of the roof panel on the basis of the respective detection outputs of the front and rear vehicle height sensors 23, 24 and controls an alarm means 29 on the basis of the calculated value and the respective detection outputs of the front and rear interval sensors 26, 27.