An object of the invention is to embody a small and inexpensive passage selector, which can be applied by commonly using an index device even if the number of detector passages is changed, and which is easy to make inspection and maintenance. The passage selector of a reactor in-core nuclear-measuring apparatus of the invention includes: a drive motor; an index device that is driven by the drive motor and that makes a rotary output of a predetermined index number; a central rotating shaft that is driven to rotate by the index device and that causes a passage selecting guide tube to be located in opposition to any detector passage; and a speed-increasing and decreasing device that is interposed between an output shaft of the index device and the central rotating shaft, and that adjusts the index number of the central rotating shaft.