The position detecting device includes a travel speed detecting unit, an optical flow calculating unit, a distance calculating unit, and a position detecting unit. The travel speed detecting unit detects a travel speed of a mobile object moving on an arbitrary route. The optical flow calculating unit calculates an optical flow of one side of the mobile object, based on a series of frame images of the one side. The one side is any one of a left side and a right side of the mobile object.The distance calculating unit calculates a distance to a subject on the one side based on the travel speed and the optical flow. The position detecting unit detects a current position of the mobile object based on point data of the subject and the distance to the subject.