An authenticator apparatus which makes it difficult for an unauthorized user to masquerade and enhances safety includes an authenticating information holding unit (102) previously stores characteristic information indicating an input and output characteristic involving an environment change of an authentic authenticatee apparatus entitled to be authentic, an authenticating information transmitting unit (107) which transmits authenticating information to a portable medium (2), a response information receiving unit (108) which receives response information outputted from the portable medium (2) in response to an input of the authenticating information, an environment selecting unit (105) which identifies an environment of the portable medium (2), and a response information confirming unit (109) which determines whether or not the authenticating information and the response information satisfy the input and output characteristic indicated in the characteristic information stored in the authenticating information holding unit (102), and judges that the portable medium (2) is authentic in the case where the input and output characteristic is satisfied, the authenticating information and the response information being in the environment identified by said environment identifying unit.