Provided are a method and a device for quantum key distribution. The quantum key distribution device may be connected to another quantum key distribution device through a quantum channel and distribute a quantum cryptography key. The device may include an optical unit having an optical element to transmit information, and a post-processor for calculating a quantum cryptography key by processing a random number generated by the optical unit. The device may include at least one of the optical unit including a plurality of unit optical modules, the post-processor having a plurality of unit post-processing modules, and the optical unit having a plurality of unit optical modules and the post-processor having a plurality of unit post-processing modules.