An electronic key registration method for registering a plurality of electronic keys (2a, 2b) to a controller of a communication subject through communication with a data center, wherein the electronic keys (2a, 2b) are capable of communicating with the controller, the electronic key registration method comprising: storing communication subject identification information and a communication subject encryption key, which are unique to the communication subject, in the controller; storing the communication subject identification information and the communication subject encryption key in the data center; storing first key identification information (KID-1) and a first key encryption key (K-1), which are unique to a first electronic key, in the first electronic key; storing second key identification information (KID-2) and a second key encryption key (K-2), which are unique to a second electronic key, in the second electronic key; storing the first and second key identification information (KID-1, KID-2) and the first and second key encryption keys (K-1, K-2) in the data center; acquiring the first key identification information (KID-1) from the first electronic key with the controller; acquiring the second key identification information (KID-2) from the second electronic key with the controller; transmitting both of the first and second key identification information (KID-1, KID-2) together with the communication subject identification information from the controller to the data center; generating a first encryption key generation code (SC-A1) with the data center, wherein the first encryption key generation code (SC-A1) is based on the communication subject encryption key and the first key encryption key (K-1); generating a second encryption key generation code (SC-A2) with the data center, wherein the second encryption key generation code (SC-A2) is based on the communication subject encryption key and the second key encryption key (K-2); obtaining both of the first and second encryption key generation codes (SC-A2, SC-A2) from the data center with the controller; generating the first key encryption key (K-1) based on the first encryption key generation code (SC-A1) with the controller; generating the second key encryption key (K-2) based on the second encryption key generation code (SC-A2) with the controller; and storing the first and second key encryption keys (K-1, K-2) in the controller. storing communication subject identification information and a communication subject encryption key, which are unique to the communication subject, in the controller; storing the communication subject identification information and the communication subject encryption key in the data center; storing first key identification information (KID-1) and a first key encryption key (K-1), which are unique to a first electronic key, in the first electronic key; storing second key identification information (KID-2) and a second key encryption key (K-2), which are unique to a second electronic key, in the second electronic key; storing the first and second key identification information (KID-1, KID-2) and the first and second key encryption keys (K-1, K-2) in the data center; acquiring the first key identification information (KID-1) from the first electronic key with the controller; acquiring the second key identification information (KID-2) from the second electronic key with the controller; transmitting both of the first and second key identification information (KID-1, KID-2) together with the communication subject identification information from the controller to the data center; generating a first encryption key generation code (SC-A1) with the data center, wherein the first encryption key generation code (SC-A1) is based on the communication subject encryption key and the first key encryption key (K-1); generating a second encryption key generation code (SC-A2) with the data center, wherein the second encryption key generation code (SC-A2) is based on the communication subject encryption key and the second key encryption key (K-2); obtaining both of the first and second encryption key generation codes (SC-A2, SC-A2) from the data center with the controller; generating the first key encryption key (K-1) based on the first encryption key generation code (SC-A1) with the controller; generating the second key encryption key (K-2) based on the second encryption key generation code (SC-A2) with the controller; and storing the first and second key encryption keys (K-1, K-2) in the controller.