PURPOSE: To easily determine the base arrangement of a DNA specimen with high accuracy, by photoelectrically reading the autoradiography due to the photographic photosensitive material of the predetermined separated and developed lines of the DNA specimen while applying predetermined processing to the digital signal thereof.

CONSTITUTION: Positional informations due to radioactive labels of specific split, decomposed and migrated lines 1)G+A+T+C, 2)G, 3)A, 4)7 and 5)C of a DNA specimen are transferred to a photographic photosensitive material 1. This photosensitive material 1 is mounted to a rotary drum 2 moving the axial direction thereof and the pervious light to laser beam 5 is detected through a mirror 3 by a photoelectric multiplier 7. The output thereof is processed with a signal processing circuit 10 through an amplifier 8 and an AD converter 9. By performing interpolation identification due to a curve fitting method to the arrangement constituted by holding the separated and developed lines 2), 3) and 4), 5) between reference lines 0), the base arrangement of the DNA specimen is easily determined with high accuracy.