PURPOSE: To specify the position of the vehicle even when there is a slanting surface on the travel path of the vehicle and to make the vehicle travel by itself with high certaintity by providing a tilt detecting means which detects the vehicle slanting.

CONSTITUTION: When the vehicle reaches the slanting surface R1 from a horizontal surface R, a level 10 operates to detect the title of the slanting surface R1. A controller 4B controls the operation of a motor on the basis of the deteted value so that the light projection direction of a laser light projecting and photodetecting device 4a is horizontal. The laser light projecting and photodetecting device 4a projects a laser beam on a reflector 5a, and consequently the accurate position of the vehicle 4 in the horizontal direction is measured. The controller 4b compares the position of the vehicle 4 calculated through the measurement with the position of the vehicle 4 found from travel information on an encorder and corrects the travel information of the encoder if there is an error to corrent the position of the vehicle 4.