PURPOSE: To easily achieve high position accuracy of an active element and a passive element relative to each other by a method wherein the active and passive elements constituting a detector are mounted to a common jig and then attached to separate two members moving relatively to each other.

CONSTITUTION: Magnets and magnet sensing switches 10 constituting a detector are attached to supports 11, 12, respectively. The supports 11, 12 are mounted to a common jig 15 so as to become one piece. Then, the respective supports 11, 12 are secured to corresponding measured parts, and thereafter the jig 15 is removed out of grooves formed in the both supports 11, 12. Upon this, installation of the detector is completed.