An RFID-tag reading/writing method of an RFID tag having an IC chip and an RFID antenna, comprising the steps of: transferring the RFID tag being capable of wireless data communication in a transfer direction from an upstream side toward a downstream side, and processing a wireless data communication by a first device antenna, a second device antenna and a third device antenna sequentially provided in the transfer direction; and calculating a reading/writing processing time based on a length for the wireless data communication of the first device antenna, the second device antenna and the third device antenna in the direction of the transfer respectively and a transfer speed of the RFID tag transferred in the transfer direction, within the reading/writing processing time the wireless data communication is executed by the first device antenna, the second device antenna and the third device antenna respectively; characterized by the steps of reading an identification number of the RFID tag by the first device antenna; writing (S6, S15) a predetermined data by the second device antenna to the RFID tag having the identification number read by the first device antenna; determining whether the process of writing to the RFID tag by the second device antenna is properly processed; writing again to the RFID tag by the second device antenna in case that the process of writing is determined not to be properly processed and that a half of the reading/writing processing time has not elapsed since the start of the process of the writing by the second device antenna; controlling of an execution of writing to the RFID tag having the identification number by the third device antenna in case that in the process of the determining the process of the writing is determined not to be properly processed and that a half of the reading/writing processing time has elapsed since the start of the process of the writing by the second device antenna. transferring the RFID tag being capable of wireless data communication in a transfer direction from an upstream side toward a downstream side, and processing a wireless data communication by a first device antenna, a second device antenna and a third device antenna sequentially provided in the transfer direction; and calculating a reading/writing processing time based on a length for the wireless data communication of the first device antenna, the second device antenna and the third device antenna in the direction of the transfer respectively and a transfer speed of the RFID tag transferred in the transfer direction, within the reading/writing processing time the wireless data communication is executed by the first device antenna, the second device antenna and the third device antenna respectively; characterized by the steps of reading an identification number of the RFID tag by the first device antenna; writing (S6, S15) a predetermined data by the second device antenna to the RFID tag having the identification number read by the first device antenna; determining whether the process of writing to the RFID tag by the second device antenna is properly processed; writing again to the RFID tag by the second device antenna in case that the process of writing is determined not to be properly processed and that a half of the reading/writing processing time has not elapsed since the start of the process of the writing by the second device antenna; controlling of an execution of writing to the RFID tag having the identification number by the third device antenna in case that in the process of the determining the process of the writing is determined not to be properly processed and that a half of the reading/writing processing time has elapsed since the start of the process of the writing by the second device antenna.