A system for using a communication terminal, comprising: a communication terminal having an identification information storage means for storing user identification information and including a contactless proximity communication function; and an operation device mounted in a vehicle and configured to perform a predetermined operation according to the user identification information, wherein the operation device includes; reading means configured to read the user identification information stored in the identification information storage means using the contactless proximity communication function; a control ECU for determining the instruction content of the predetermined operation for instructing the vehicle based on the user identification information read by the reading means and the length of the communication time between the reading means and the communication terminal; and a body ECU configured to perform a lock release operation releasing a locked lock on a locked door of the vehicle in response to a lock release signal transmitted from the control ECU or a locking operation locking the door of the vehicle in response to a lock signal transmitted from the control ECU; wherein the control ECU is configured to determine between instruction content that corresponds to a predetermined first communication time and instruction content that corresponds to a second communication time longer than the first communication time; wherein the body ECU is configured not to instruct a usage starting and stopping operation of the vehicle corresponding to the second communication time until instruction of a door locking operation of the vehicle corresponding to the first communication time is performed; and wherein the body ECU is configured to instruct the usage starting and stopping operation of the vehicle after the instruction of the door locking operation of the vehicle is performed. a communication terminal having an identification information storage means for storing user identification information and including a contactless proximity communication function; and an operation device mounted in a vehicle and configured to perform a predetermined operation according to the user identification information, wherein the operation device includes; reading means configured to read the user identification information stored in the identification information storage means using the contactless proximity communication function; a control ECU for determining the instruction content of the predetermined operation for instructing the vehicle based on the user identification information read by the reading means and the length of the communication time between the reading means and the communication terminal; and a body ECU configured to perform a lock release operation releasing a locked lock on a locked door of the vehicle in response to a lock release signal transmitted from the control ECU or a locking operation locking the door of the vehicle in response to a lock signal transmitted from the control ECU; wherein the control ECU is configured to determine between instruction content that corresponds to a predetermined first communication time and instruction content that corresponds to a second communication time longer than the first communication time; wherein the body ECU is configured not to instruct a usage starting and stopping operation of the vehicle corresponding to the second communication time until instruction of a door locking operation of the vehicle corresponding to the first communication time is performed; and wherein the body ECU is configured to instruct the usage starting and stopping operation of the vehicle after the instruction of the door locking operation of the vehicle is performed.