PURPOSE: To make adjustable the spacing of a part for measuring luminous intensity according to conditions by a construction which makes it possible to change arbitrarily the pitch of arrangement of the end faces of optical fiber wires of an optical fiber array on the side on which rays of light of an image of an object are incident.

CONSTITUTION: The image of a surface of an object 1 taken by a first lens 2 is formed on the end faces of one side of the respective optical fiber wires of an optical fiber array 7and transmitted to the respective end faces on the other side of the wires. Moreover, it is formed on the light-receiving faces of the respective sensors of a sensor array 5 by a second lens 4, converted into an electric signal by each sensor, and sent to a signal processing circuit 6. In the case when a device thus constructed is employed for detecting the width of a hot- rolled sheet in a hot-rolling line of ironworks, the pitch of arrangement of the end faces of the respective optical fiber wires of the optical fiber array 7 on the first lens 2 side is set so that it is close in the end parts in the width of the sheet and rough in the central part thereof in order to attain a luminous intensity close in spacing in the vicinity of the end parts in the width of the sheet and thereby to improve the accuracy in detection.