This paper describes an in-body/out-body passive type RFID system by using a miniaturized tag antenna. The RFID tag is used for both in-body and out-body communications. In the out-body condition, the tag antenna is designed at operating frequency 5.02 GHz in order to shrink the tag size to about 2cm length. In the in-body condition, the operating frequency becomes lower than that in out-body condition due to the higher dielectric constant of surrounding medium (i.e. human body and liquid) of the antenna. From the EM simulation result, we found that this tag can be operated at 920 MHz-band which is commonly used in existing 920 MHz RFID system. Therefore, we decided to use existing 920 MHz RFID tag chip and fabricated the miniaturized RFID tag. Fabricated 2cm-class RFID tag performs dual-use characteristic for in-body/out-body conditions by using 5.02 GHz/920 MHz bands.