A password management system includes a character string conversion unit that converts an input character string to a character string different from the input character string using a predetermined arithmetic expression, the predetermined arithmetic expression being capable of giving a same converted result for a same character string and capable of converting a character string such that a plurality of original character string candidates exist for a converted character string; a password data storage unit that stores, for each user, a converted past password character string CPP that is a converted state of a past password character string PP of the user using the predetermined arithmetic expression and is different from the past password character string PP; and a strength determining unit that determines strength of a password.