PURPOSE: To perform an easy work of measurement for the position of an electron gun inserted into a cathode-ray tube misaligned with respect to the central axis of the tube, by utilizing contactless displacement meters arranged at a face-to-face position to each other in the outside of the tube with a prescribed distance apart from the central axis of the tube.

CONSTITUTION: An electron gun 12 is supported onto a freely movable table 11 and inserted into a neck part 13 of a cathode-ray tube. In the outside of the neck part 13 contactless displacement meters 14, 15 are arranged at a face-to-face position to each other by using mounting plates 16, 17 in a position with an equal distance C, C' from a central axis 18 of the neck part 13. Then because each of distance A, B between the displacement meters 14, 15 and the electron gun is measured as outputs P1, P2, the correction of the eletron gun 12 to a position aligned to the central axis 18 can be performed by moving the table 11 in such a manner that the distances A, B, that is, the outputs P1, P2 are equal. While a use of one contactless displacement meter while being given a turning movement around the neck part 13 can also perform a detection of misaligned positioning.