A secret communication method and a communication device used in the method are provided for secret communication using communication path less frequently as a whole while avoiding a duplicative use of public communication. The secret communication method includes steps of: estimating an error ratio of initial random numbers X, Y; estimating the upper limit of an eavesdrop information amount; determining an encryption function determined by the error correction code based on the estimated error probability, an error correction decoding function g, and a decryption auxiliary variable; uniquely determining a confidentiality increase matrix C based on the estimated upper limit value of the eavesdrop information amount and the encoding ratio of the error correction code; uniquely generating a ciphertext Z from information M to be sent to a receiver using the encryption function, the initial random number X, and the confidentiality increase matrix C; transmitting the ciphertext Z; and decrypting the ciphertext Z to a transmission text MB using the initial random number Y, the confidentiality increase matrix C, the decryption auxiliary variable, and the error correction decoding function g.