PURPOSE: To easily discriminate a machine body number, by arranging black spots in the form of a matrix on the outside of a body case, and reading them optically.

CONSTITUTION: On a white top surface plate 13 of a measuring machine body 10, black spots are provided in the form of a matrix. As for the black spots, a circular dot-like through-hole 15 is provided on a necessary spot of the top surface plate 13 and is closed by a black sheet 17. For instance, in case of a matrix consisting of 5 lines and 6 rows, a start code and a synchronizing mark are defined by the first row R1 and the fifth line L5, respectively, the matrix is formed by the second row R2W the sixth row R6, and the first line L1W the fourth line L4, digits 100W104 are displayed by R2WR6, and "1" and "0" of a binary code are displayed by L1WL4. In case of the figure, 4×104, 3×103, 2×102, 1×101, 1×100=43,211 is displayed. In this way, the machine body number can be read exactly by moving the photodetector and the light emitting device in the direction as indicated with an arrow (a), a starting them by R1, synchronizing them by L5, and detecting whether a reflection exists or not. This system is applied to a dosage meter and is suitable for processing an exposed radiation quantity of each person.