A sensor network includes a sensor management server which is mutually connected with many sensor terminals. The sensor management server includes: a key delivery module for transmitting a key update message to the respective sensor terminals; a communication volume detection module for detecting a communication volume used for transmitting the key update message; and a multiplicity determination module for specifying a numerical value as a multiplicity, which represents the lower limit number of key update messages transmitted to the respective sensor terminals, according to the detected communication volume. The multiplicity determination module specifies the multiplicity such that the communication volume for transmitting or retransmitting the key update message takes the minimum value, and the key delivery module transmits the key update message by using the minimum communication volume that corresponds to the specified multiplicity.