To improve security of a magnetic card.

A host makes a veracity determining proper number on the basis of a dynamic parameter such as the date, a minute, and a second, and records the proper number on both the magnetic card 10 and the host. When using the card, a reader determines truth by checking both, and renews both by newly making a proper number by a similar method as before. A thief having malice becomes extremely difficult to analogize a proper number making method, by using the dynamic parameter. Since the proper number is renewed with every card use, even if the making method can be analogized, theft is actually difficult.