PURPOSE: To improve accuracy, by arranging a code plate having specified patterns so as to face an optical sensor group of a body at the upper part of a spring with the plate located under the spring, and detecting the amount of relative displacement of the spring in the up and down direction based on the light, which is projected from light sensors and reflected by the code plate.

CONSTITUTION: A light sensor case 12, which has light emitting elements 14 and light receiving elements 16, is mounted on the outer surface of a strut 10, which is linked to a body on a spring. A code plate 20 is mounted on the inner surface of a dust cover 18, which is linked to a suspension mechanism on the lower side of the spring so as to face the elements 14 and 16. Reflecting and non-reflecting coded patterns are arranged on the plate 20 so as to correspond to the arrangement of light sensors comprising the elements 14 and 16. The width l of the pattern is determined to be the length, which allows the displacement of each light sensor in the arranged direction within a specified range. Since the case 12 and the plate 20 can be arranged without contact, duribility can be improved, and the detection can be performed even though each light sensor is displaced up to the width l.