a key system includes a first authentication part configured to perform first authentication for determining whether or not a unique key code of a key is registered in the operation subject and to permit an operation subject to operate in a state in which a first group of functions is operative when the first authentication generates an affirmative result . a wireless signal receiver is configured to receive a medium code unique to a portable communicable data medium through wireless communication . a second authentication part is configured to perform second authentication for determining whether or not the medium code received by the wireless signal receiver is registered in the operation subject . a condition - imposing part is configured to limit operative functions of the operation subject to a second group of functions that is more limited than the first group of functions when at least either one of the first authentication and the second authentication generates a negative result .