1. An order-preserving encryption system, comprising: an encryption device which encrypts an inputted plaintext and outputs an encrypted text; and a decryption device which decrypts the inputted encrypted text and outputs the plaintext, wherein the encryption device comprises: a storage module which stores in advance an encryption key required for encryption processing; a pre-processing function unit which applies, on the plaintext, a pre-processing function for converting input values that may not generally be distributed uniformly to output values distributed uniformly; and an encryption unit which outputs the encrypted text that is acquired by encrypting the plaintext on which the pre-processing function is applied with order-preserving encryption by using the encryption key.