In this paper, a bistable laser-diode memory made of an APD, an LD and an optical feedback circuit is studied from the point of low set power and high speed. To attain a low set power, the effect of spontaneous emission of an LD cannot be neglected. Therefore, the input and output characteristics of an optical memory are studied including the spontaneous emission coefficient. Next, the response speed at a low input power is studied and the relationship between the response speed and the optical memory design parameters is derived. The input-output characteristics and the response speed are verified with an experimental circuit. Finally, a high-speed optical memory with a low set power is fabricated. The results of operating experiments of optical memories in an optical switching network are described.