as shown in a circuit diagram of fig1 , a transceiver 200 of a communication system of a preferred embodiment of the present invention comprises an encoder 30 , a transmission amplifier 32 , an environment - side electrode 34 , a living body - side electrode 36 , a decoder 40 , a reception amplifier 42 , a changeover switch 44 , and a controller 46 . the transceiver 200 can be incorporated into a portable electronic device such as a portable phone and a pda , and used as a portable communication terminal . in addition , the transceiver 200 may be placed in infrastructures such as a ticket barrier in a station , a vending machine , a shop , etc ., and used as a communication terminal . for example , users carrying portable communication terminals on each of which the transceiver 200 is mounted may contact or closely place a part of the body , to exchange information between the transceivers 200 . in addition , a user carrying the portable communication terminal on which the transceiver 200 is mounted may contact or closely place a part of the body to the living body - side electrode 36 of the transceiver 200 placed in an infrastructure such as a ticket barrier , to exchange data between the transceivers 200 . the encoder 30 encodes data which is input from the outside using a predetermined encoding method and outputs the encoded data to the transmission amplifier 32 . the data which is input from the outside is input in a superposed state on a base wave of a high frequency . the frequency of the base wave is preferably set , for example , to 5 mhz or higher or 15 mhz or lower . the transmission amplifier 32 comprises a differential amplifier circuit . an inverted input terminal (−) and a non - inverted input terminal (+) of the differential amplifier circuit are connected to an output terminal of the encoder 30 . an inverted output terminal and a non - inverted output terminal of the transmission amplifier 32 are connected to the environment - side electrode 34 and the living body - side electrode 36 through the changeover switch 44 . in the state where the inverted output terminal and the non - inverted output terminal of the transmission amplifier 32 are connected to the environment - side electrode 34 and the living body - side electrode 36 by the changeover switch 44 , the transmission amplifier 32 amplifies a signal which is input from the encoder 30 and differentially outputs to the environment - side electrode 34 and the living body - side electrode 36 . a potential difference between the environment - side electrode 34 and the living body - side electrode 36 changes according to the output of the transmission amplifier 32 . the environment - side electrode 34 and the living body - side electrode 36 are each formed with a conductor . for example , these electrodes are formed in a sheet shape or a plate shape with a conductor such as aluminum , stainless steel , copper foil , copper sheet , etc . the environment - side electrode 34 and the living body - side electrode 36 are preferably placed in a state electrically insulated from each other , sandwiching a dielectric layer 38 . the reception amplifier 42 comprises a differential amplifier circuit . an inverted input terminal (−) and a non - inverted input terminal (+) of the differential amplifier circuit are connected to the environment - side electrode 34 and the living body - side electrode 36 through the changeover switch 44 . an inverted output terminal and a non - inverted output of the differential amplifier circuit are connected to an input terminal of the decoder 40 . in the state where the inverted input terminal (−) and the non - inverted input terminal (+) of the reception amplifier 42 are connected to the environment - side electrode 34 and the living body - side electrode 36 by the changeover switch 44 , the reception amplifier 42 amplifies a potential difference between the environment - side electrode 34 and the living body - side electrode 36 and outputs as a potential difference between the inverted output terminal and the non - inverted output terminal . the decoder 40 receives an output signal from the reception amplifier 42 , decodes the signal using a decoding method corresponding to the encoding method used in the encoder 30 , and outputs the decoded signal . in the transceiver 200 , the environment - side electrode 34 and the living body - side electrode 36 are shared between transmission and reception . the controller 46 switches the changeover switch 44 to the transmission amplifier 32 when data is to be transmitted and switches the changeover switch 44 to the reception amplifier 42 when data is to be received . the timing of the switching of the changeover switch 44 may be at a predetermined time period or may be achieved by the user . for example , as shown in fig2 , a communication session can be executed by executing a polling process . in the following description , the transceivers 200 provided in the fixed device and the portable device periodically switch between the transmission state and the reception state at a predetermined period . the transceiver 200 which is at a transmission side sends a call to the transceiver 200 which is at a reception side . when there is no transceiver 200 of the reception side in the reception range of the calling signal from the transceiver 200 of the transmission side , the call is repeated . when , on the other hand , the transceiver 200 of the reception side can receive the calling signal from the transceiver 200 of the transmission side , the transceiver 200 of the reception side receiving the calling signal is activated . the activated transceiver 200 of the reception side transmits a connection request signal to the transceiver 200 of the transmission side . when the transceiver 200 of the transmission side receives the connection request signal , the transceiver 200 of the transmission side returns a connection response signal if connection is possible . when the transceiver 200 of the reception side receives the connection response signal , the transceiver 200 of the reception side returns a connection response signal to the transceiver 200 of the transmission side . when the transceiver 200 of the transmission side receives the connection response signal , a data communication session is started . when the communication of data is completed , a completion signal is transmitted from the transceiver 200 of the transmission side to the transceiver 200 of the reception side , and the transceiver 200 of the reception side enters a sleep mode . alternatively , the transceiver 200 may have a structure as shown in fig3 . in this structure , the transmission amplifier 32 comprises a single amplifier circuit . an output terminal of the encoder 30 is connected to an input terminal of the amplifier circuit , and an output terminal of the amplifier circuit is connected to the living body - side electrode 36 through the changeover switch 44 . the environment - side electrode 34 is grounded . in the state where the output terminal of the transmission amplifier 32 is connected to the living body - side electrode 36 by the changeover switch 44 , the transmission amplifier 32 amplifies a signal which is input from the encoder 30 and outputs the amplified signal to the living body - side electrode 36 . a potential difference between the environment - side electrode 34 and the living body - side electrode 36 changes according to an output of the transmission amplifier 32 . the reception amplifier 42 comprises a single amplifier circuit . an input terminal of the amplifier circuit is connected to the living body - side electrode 36 through the changeover switch 44 , and an output terminal of the amplifier is connected to the decoder 40 . the environment - side electrode 34 is grounded . in the state where the input terminal of the reception amplifier 42 is connected to the living body - side electrode 36 by the changeover switch 44 , the reception amplifier 42 amplifies a signal which is input from the living body - side electrode 36 and outputs the amplified signal to the decoder 40 . the decoder 40 decodes the output of the reception amplifier 42 using a decoding method corresponding to the encoding method used in the encoder 30 and outputs the decoded signal . the transceiver 200 of the present embodiment is mounted in a housing 50 as shown in fig4 . such a configuration is particularly preferable , for example , when the transceiver 200 is used as a portable communication terminal such as a portable phone and a pda . in fig4 , electronic circuits used in the transceiver 200 are mounted on a circuit board 52 . in the present embodiment , at least the reception amplifier 42 is mounted on the circuit board 52 . for example , the encoder 30 , the transmission amplifier 32 , the decoder 40 , the reception amplifier 42 , the changeover switch 44 , and the controller 46 are mounted on the circuit board 52 . here , the environment - side electrode 34 , the living body - side electrode 36 , and the circuit board 52 are placed in the housing 50 . in the present embodiment , as shown in a perspective view of fig5 , the circuit board 52 is placed not in a location sandwiched between the environment - side electrode 34 and the living body - side electrode 36 . when the environment - side electrode 34 and the living body - side electrode 36 are plate - shaped electrodes and are placed opposing each other in a manner where at least a part of the electrodes overlap each other , the circuit board 52 is preferably placed at a position where the circuit board 52 is spatially hidden by at least a part of one of the electrodes from at least a part of the other electrode . for example , as shown in fig1 a , preferably , a configuration is employed in which the circuit board 52 , the environment - side electrode 34 , and the living body - side electrode 36 are set to approximately the same size and the circuit board 52 is spatially hidden by one of the electrodes from the other electrode . alternatively , as shown in fig1 b , preferably , a configuration is employed in which the environment - side electrode 34 and the living body - side electrode 36 are set to approximately the same size , the circuit board 52 is set to a size smaller than the environment - side electrode 34 and the living body - side electrode 36 , and the circuit board 52 is spatially hidden by one of the electrodes from the other electrode . alternatively , as shown in fig1 c , preferably , a configuration is employed in which one of the environment - side electrode 34 and the living body - side electrode 36 which is placed closer to the circuit board 52 is set to approximately the same size as the circuit board 52 , the other electrode is set to a size smaller than the circuit board 52 , and the circuit board 52 is spatially hidden by the one electrode from the other electrode . alternatively , as shown in fig1 d , preferably , a configuration is employed in which the environment - side electrode 34 and the living body - side electrode 36 are set to approximately the same size , the circuit board 52 is set to a size larger than the environment - side electrode 34 and the living body - side electrode 36 , and at least a part of the circuit board 52 is spatially hidden by one of the electrodes from the other electrode . in a configuration as shown in fig1 e where one of the environment - side electrode 34 and the living body - side electrode 36 which is closer to the circuit board 52 is set to a size smaller than the circuit board 52 and the other electrode is set to a size larger than the circuit board 52 , on the other hand , there is a portion where the circuit board 52 is not spatially hidden by one of the electrodes from the other electrode , and thus the obtained advantage is lower . by relatively placing the environment - side electrode 34 , the living body - side electrode 36 , and the circuit board 52 in a manner described above , as shown in fig6 , it is possible to block the circuit board 52 by one of the environment - side electrode 34 and the living body - side electrode 36 , set a capacitive coupling c 4 between the other electrode and the circuit board 52 to be smaller than the parasitic capacitances c 1 and c 2 in the structure of related art , and reduce the influence of the electromagnetic noise from the circuit board 52 on the other electrode . for example , in a circuit structure which shows reception signals of fig1 a and 16b , if the structure is changed to a structure as in the present embodiment where the circuit board 52 is not sandwiched between the environment - side electrode 34 and the living body - side electrode 36 , a reception signal as shown in fig7 is obtained . the reception signal obtained in the structure of the present embodiment has a higher absolute intensity than the reception signal of fig1 a and an improved s / n ratio over the reception signal of fig1 b . as described , by employing the structure of the transceiver 200 of the present embodiment , it is possible to improve the intensity of the reception signal at the reception and to improve s / n ratio of the reception signal . as shown in fig1 a , the electrode which is placed nearer to the circuit board 52 may be set as the environment - side electrode 34 and the other electrode may be set as the living body - side electrode 36 . alternatively , as shown in fig1 b , the electrode placed nearer to the circuit board 52 may be set as the living body - side electrode 36 and the other electrode maybe set as the environment - side electrode 34 . in these configurations , the phase of the reception signal during usage would be inverted . however , by applying a modulation method which does not depend on phase inversion such as psk , fsk , and ask , it is possible to use in the reverse direction . in other words , the names of the environment - side electrode 34 and the living body - side electrode 36 are only for the purpose of convenience , and the electrodes may be used in the opposite configuration . as shown in fig1 , the dielectric layer 38 which functions as an antenna and which is sandwiched between the environment - side electrode 34 and the living body - side electrode 36 is not limited to a resin such as epoxy , and an insulating material similar to the circuit board 52 or a foam resin having a low dielectric constant may be used . when the housing of the electronic device such as the portable phone is made of an insulating member , as shown in fig2 , a structure may be employed in which the housing itself is used as the dielectric layer 38 and sandwiched by the environment - side electrode 34 and the living body - side electrode 36 . in this case , it is also preferable to cover the electrode which is placed outside of the housing with an insulating film 54 . with such a structure , it is possible to improve the degree of freedom of placement of the members in the housing . in the present embodiment , a transceiver 200 having both the transmission system and reception system is exemplified , but the present invention is not limited to such a configuration , and may be applied to a communication system in which a transmitter of a transmission system and a receiver of a reception system are separately formed . specifically , as shown in fig8 , in a receiver 202 comprising an environment - side electrode 44 , a living body - side electrode 46 , a decoder 40 , and a reception amplifier 42 , the circuit board may be provided not sandwiched between the environment - side electrode 44 and the living body - side electrode 46 , to obtain similar advantages . alternatively , as shown in fig9 , a configuration may be employed in which the reception amplifier 42 is changed from the differential amplifier circuit to a single amplifier circuit .