An object of the invention is to provide a system for detecting the position of an underwater vehicle that enables an improvement in the accuracy of detecting the position of the underwater vehicle.The position detection system comprises: a pressure sensor 18 that detects the vertical position of an underwater vehicle 9; a range sensor unit 23 that detects the relative distances of the underwater vehicle 9 from its surrounding structures; a measurement image acquisition unit 36 that acquires a measurement image of the horizontal plane in which the relative distances have been detected; an image storage unit 34 that stores images, each of which has at least one pixel to which horizontal position information is attached and is the image data representing the outer shapes of the structures in a given horizontal plane; an image selector 35 that selects, based on the vertical position of the underwater vehicle 9, one of the stored images that corresponds to the horizontal plane in which the relative distances have been detected; a corresponding-area identification unit 37 that identifies the area in the selected image that corresponds to the measurement image by performing map matching; and horizontal position calculator 38 that identifies, from the corresponding area in the selected image identified by the corresponding-area identification unit 37, the pixel that corresponds to the position at which the relative distances have been detected and calculates the horizontal position of the underwater vehicle.