Position detection method and position detection device

In the present invention, deviation between the origin for the measuring coordinate of a master device for detecting a slave device for flow detection such as of a pressure vessel of a nuclear reactor and a reference point disposed to the pressure vessel of the nuclear reactor. That is, one end of the arm is secured to a rotational shaft of an index table rotatably disposed to the master device. A vertical direction sensor is disposed to the index table for detecting that the positive direction of a vertical axis of the measuring coordinate system is upwardly set vertically. A camera is disposed at a position displaced from the measuring coordinate system for displaying the reference point on the wall surface on which the master device is disposed. The arm is rotated, so as to align the center line of the arm with the origin of the coordinate system of the camera at which the reference point is displayed. With such a constitution, each of inclined angles of the coordination system of the arm and the camera and the measuring coordinate system can be detected. Based on the inclined angles and the distance between both of the origins of the coordinates system, a deviation between the reference point formed to the reactor pressure vessel and the origin of the measuring coordinate system in detected. (I.S.)