To detect the position of a mobile object without using a specified inspection point of a position mark or an illumination device or the like.

The mobile object 21 is provided with electronic cameras 2a and 2b capable of picking up the image of a ceiling surface and a light 4 to be an auxiliary light source at the time of picking up the image by using the electronic cameras on an upper part. The mobile object incorporates a position detection means 1 for detecting its own position. The position detection means compares the image for which the ceiling surface is extracted and which is stored beforehand with the image of the ceiling surface extracted from the image photographed by using the electronic camera and detects the position of the mobile object.