The present invention relates to an apparatus and a method for realizing all-optical NOR logic device using the gain saturation characteristics of a semiconductor optical amplifier (SOA). More particularly, the invention relates to a 10 Gbit/s all-optical NOR logic device among all-optical logic devices, in which a signal transmitted from a given point of an optical circuit such as an optical computing circuit is used as a pump signal and a probe signal. The method for realizing an all-optical NOR logic device using the gain saturation characteristics of the SOA according to the present invention comprises the steps of: utilizing A+B signal which couples together an input signal pattern A (1100) and an input signal pattern B (0110) as a pump signal (1110); utilizing a probe signal (1111) by generating a clock signal out of said input signal pattern A (1100); and obtaining a Boolean equation {overscore (A+B)} by making said probe signal and said pump signal incident upon the SOA simultaneously from the opposite direction. The all-optical logic device according to the present invention has a simple construction since it is realized through the XGM (Cross Gain Modulation) method which utilizes the gain saturation characteristics. Also, it is expected that the method employed in the present invention could be used for realizing other all-optical logic circuits and devices. Key Words 10 Gbit/s, All-optical NOR logic device (10 Gbit/s All-optical NOR logic device), Semiconductor Optical Amplifiers, Gain Saturation