A portable device includes an RFID reader for detecting a tag information of an RFID tag, a memory for storing the tag information detected by the RFID reader, and a controller. When a read command for the RFID tag is input and the tag information is valid, the controller controls the RFID reader to retrieve the tag information of the memory. If the tag information is invalid, the controller controls the RFID reader to re-read the RFID tag. The controller can determine whether the tag information is valid using the time and location. Because unnecessary operation can be omitted to read tags, power consumption is reduced and tag information check time is shortened.