Abstract: A high speed optical analog/digital conversion process with a simple configuration, in which an optical-pulse branching delay multiplexer generates as many duplicates of each optical pulse as the predetermined number of bits by branching each optical pulse string obtained by sampling an input optical analog signal, and generates a temporally consecutive optical pulse string by multiplexing the duplicated optical pulses with a predetermined intensity difference and time difference. An optical encoder and an optical-threshold processing unit perform an encoding and a quantization of an optical pulse string, and output an optical digital signal corresponding to the optical analog signal.