When a detector sensor such as an infrared sensor or the like detects a palette or cart mounting a product, the output of the radio wave from the antenna of the reader/writer is set to a minimum value. Then, the radio wave output value is gradually increased from the minimum value until an RFID tag can be read/written. After the RFID tag can be read/written, a target process such as the reading/writing of the RFID tag or the like is performed and the process is terminated. After the process is terminated, the radio wave output is returned to the minimum value.