Noncontact data carrier

When performing radio communication with a reader/writer (1), supply of a power source S4 from a battery (14) is started when a level judgment circuit (9) judges that the signal level of a reception signal S2 received by an antenna (5) of a noncontact data carrier (4) is equal to or higher than a predetermined amplitude and a timer circuit (10) judges that the state where the amplitude is equal to or higher than the predetermined value has continued for a predetermined period T1 or more, and further, the supply of the power source S4 from the battery is stopped when the level judgment circuit (9) judges that the signal level of the reception signal S2 is lower than the predetermined amplitude and the timer circuit (10) judges that state where the amplitude is lower than the predetermined value has continued for a predetermined period T2 or more, thereby ensuring communication reliability that causes no problem in practical use of the noncontact data carrier (4), and avoiding insignificant dissipation of the battery.