PURPOSE: To surely detect the fluctuation of optical signals generated from an optical fiber sensor by an OTDR even in a long distance from the detection positions of oil and temperature, etc., from the OTDR.

CONSTITUTION: This detector of the oil and the temperature etc., detects the fluctuation of the optical signals from the optical fiber sensors 1 capable of detecting the oil and the temperature, etc., by the OTDR 2, analysis-processes it and detects the oil and the temperature, etc. The optical fiber sensors 1 are serially connected with fibers 3 for optical transmission with less transmission loss to be defined as optical detection lines 4, more than two of the optical detection lines 4 are parallelly arranged in the detection area of the oil and the temperature, etc., and the arranging position of the optical fiber sensors 1 of the optical detection lines 4 is shifted in the axial direction of the optical detection lines 4.