hereinafter , embodiments of the present invention will be described with reference to the accompanying drawings . although the following embodiments are described by using specific numeric values for easily understanding the embodiments , the numeric values are merely examples . therefore , the present invention is not limited to those numeric values . in a first embodiment , a transmitting unit and a receiving unit shown in fig1 are used as an encryption transmission apparatus employing an optical multi - value transmission method . in an optical encrypted signal transmitting unit , transmission data composed of two - value digital data before encryption is converted into multi - value signals by a digital - to - analog converter ( dac ) after scramble encoding is executed by an encoder . at a time of the conversion to the multi - value signals , a signal is generated in accordance with a logical threshold determiner connected to the dac . an output from the dac is converted into the multi - value signals at the optical transmitting unit and thereafter is sent into an optical fiber . an optical signal transmitted to the optical fiber is converted to an analog electric signal at the optical receiving unit . the electric signal from the optical receiving unit is converted to an electric signal with discriminable amplitude by an amplifier and thereafter is decoded into a two - value digital signal by a logical threshold discriminator . the logical threshold discriminator discriminates the two - value digital signal by using as a reference a logic threshold dynamically defined by the logical threshold determiner . the discriminated digital signal is outputted as a received data to the next stage after scramble codes are decoded by the decoder . an encryption method used in the present invention is the same as one described in ieee , trans . on information theory , vol . 49 , no . 12 , pp . 3312 - 3317 , 2003 . in the method in the present embodiment , a between - level voltage of the signal used for multi - value transmission is set to a level lower than that of shot noise accumulated in the signal at a time of passing through the optical transmitting unit and the optical receiving unit and dynamically fluctuates the logic threshold so as to be synchronized at a transmitting terminal and a receiving terminal in accordance with a rule well - known only by a transmitter and a receiver . therefore , a reception environment with much noise can be provided to eavesdroppers who do not know the transient rule of the fluctuated logic threshold , and a good reception environment with little noise can be provided to proper receivers who know the transient rule well . thus , since the reception environments different in level of noise can be realized depending on the eavesdroppers and the proper receivers , the eavesdroppers can decrypt only the erroneous data and the proper receivers can receive the correct signals at a time of receiving the signals with scramble codes conforming to streaming codes . the principle of operation of encryption communication used in the present invention will be described with reference to fig2 and 3 . fig2 shows a signal setting of a transmitter side . it is first assumed that 5 - bit continuous data , 11001 , is transmitted from the transmitter side . the continuous data is transmitted by a multi - value modulation in which a signal amplitude level is divided into 14 different stages . at this time , the level of the multi - value modulation is set to have a value smaller than the distribution value of shot noise . the actual transmission signal is changed to signal levels 7 , 7 , 8 , and 8 . in contrast , a logic threshold setting is changed to signal levels 4 , 4 , 11 , and 11 . if a level relation between the actual transmission signal and the logic threshold is known , the original data value , 11001 , from the transmission signal can easily be discriminated . fig3 illustrates an example of a waveform of which the signal of fig2 is received through a transmission path . the reception signal is affected by shot noise ( of which a distribution value is divided into four levels of the signal amplitude ) and values of the reception signal have a probability distribution within a range from − 2 levels to + 2 levels . due to the influence of noise , the signal is received as the multi - level modulation signal with , for example , values of 6 , 8 . 5 , 9 , 6 , and 5 . in this case , the proper receivers , who know the logic threshold setting , can introduce the transmission data signal , 11001 , on the basis of the reception signal . however , the eavesdroppers , who have no measures of knowing the logical threshold setting , must introduce the original data signal from the values of 6 , 8 . 5 , 9 , 6 , and 5 serving as reception signal pattern and further the reception signal pattern has no reproducibility due to the influence of noise at a time of transmitting the same pattern ( namely , values of 9 , 8 , 5 , 8 , and 8 may be received at a time of another transmission ). as a result , the eavesdroppers cannot receive the signal under the same qualified communication condition as that of the proper receivers ( i . e ., do not fail to receive the signal under the environment with a higher reception error rate ), so that the environment in which eavesdropping the data is impossible can be realized . as described with fig1 to 3 , a communication line for encryption is outputted by converting two - value digital data to a multi - value signal with 14 levels . in a circuit used for the conversion to the multi - value signal with 14 levels , the circuit shown in fig1 is newly needed in comparison with the case of directly outputting the two - value digital data . in addition , in order to realize the multi - value transmission with high quality , a wideband circuit with a low noise level is required , so that devices constituting the apparatus become difficult to realize and the costs of the apparatus to be realized are increased . in the communication circuit for encryption , even if such a circuit that the two - value digital signal can be transmitted at a speed of 10 gigabits per second is used , only data transmission of 100 megabits at best can be realized due to lack of band of the circuit . for this reason , instead of using the transmission and reception circuits for encryption to realize the data itself , an apparatus configuration shown in fig4 is used to use at the same time two lines for realizing encryption communication and data communication . in the apparatus configuration shown in fig4 , an optical signal transmitter for encryption and an optical signal receiver for encryption are mounted as one system , whereas the data communication system is configured by two systems . in the data communication , when transmission data 1 is transmitted , only an encryption key used for communication is transmitted by using an optical fiber 1 , the optical signal transmitter for encryption , and the optical signal receiver for encryption . the actual data is encrypted by the encryption key used for communicating with an encode 1 , and then is transmitted by using a data transmitter 1 , an optical fiber 2 , and a data receiver 1 , and is decoded at a decoder 1 by using the encryption key and is fetched from the reception data 1 . as the data transmitter 1 and the data receiver 1 , a normal two - value digital data communication apparatus is used instead of using an encryption communication apparatus . in the present invention , unrealizable data transmission at a speed of 10 gigabits per second can be realized in the encryption communication by using the two - value digital communication line . since two two - value data communication systems share one encryption communication system , the high - speed data communication can be realized at low cost in comparison with the case of making both of an exchange of the encryption key and transmission of the actual data by using the encryption communication system . fig5 illustrates a scheme of a handshaking of the actual signal transmission . first , the scheme needs to transfer the logic threshold setting from the transmitter to the receiver . however , since the transmission of the logic threshold by using the data line means to provide an opportunity for making eavesdropping of the logical threshold setting possible , the communication of the setting data by using the high - security communication path is required for transfer of the logic threshold to use an applicable high safety manner such as postal service or wire telephone service . in this embodiment , the logic threshold is transferred over the telephone line from the transmitter to the receiver which in turn issues the acknowledgment of reception before the procedure moves to the succeeding step . at the succeeding step , the encryption key is transmitted by using an encryption transmission path from the transmitter to the receiver . for transmission of the encryption key , the logic threshold determined at the preceding stage is used for dynamically varying the threshold to ensure a higher level of the secrecy . using the encryption key , the data to be transmitted is encrypted before transmitted by using the two - value digital transmission path . when a predetermined length of time ( for example , one minute ) determined by the user has elapsed , the encryption key is replaced by another which is then transmitted from the transmitter to the receiver . as the encryption key is switched from one to another at intervals of the predetermined time which is commonly faster than the time required for the eavesdroppers decoding the encryption , it can hardly be disclosed to any encrypting . the encrypted data transmission of the first embodiment involves transmission of a multi - value transmission over the optical transmission path . this causes the signal to be varied in the amplitude by the effects of thermal condition or physical disturbance and the drift characteristics of a laser transmitter or photodiode . it is hence desired to modify the threshold at the receiver side according to the modification of the signal amplitude . prior to the transmission of the encrypted data , reference signals for defining the upper and lower limits of the data are determined . fig6 illustrates an example of the waveform of the reference signals . assuming that the signal level of the multi - value has 9 levels , a reference signal 1 outputs the upper limit of signal intensity level 9 while a reference signal 2 outputs the lower limit of a signal intensity level 1 . the two reference signals 1 and 2 may be varied due to the effect of the modification characteristics of the transmission path before being received by the receiver . as the reference signals 1 and 2 are monitored by the receiver , the multi - value transmission signal level to be monitored can be set on the receiver side . this embodiment has an error detector connected at the rear side of the encryption optical signal receiver as shown in fig7 . the error detector is arranged for , when an error is found in the decoder of the encryption optical signal receiver shown in fig1 , permitting the signal not to be outputted to the rear side of the detector until the error is eliminated . the error may result from injury of the optical fiber which critically declines the intensity of the optical signal and thus causes the amplitude to be hardly measured by the receiver side or thermal drift in the optical fiber which changes the intensity of the optical signal . accordingly , the receiver may fail to receive and decode the data signal produced by the encryption rule . fig8 illustrates a procedure for changing the reference signals from the transmitter to the receiver . prior to the transmission of the encrypted data , the reference signals are transferred from the transmitter to the receiver which in turn examines the threshold . when a specific length of time has elapsed which is longer than the time required for examining the threshold , the encrypted data is transferred from the transmitter to the receiver . as a result , the receiver can decode the encrypted data through reviewing the logic threshold defined by the reference signals . the encrypt communication of the first embodiment needs to satisfy the following conditional formulas for ease of acknowledging the amplitude of the multi - value signal at the receiver ( where α1 , α2 , α3 , and α4 are coefficients not smaller than 1 or preferably equal to 1 . 5 ). ( step value in multi - value signal )× α1 & lt ;( distribution value of shot noise in jitter component of received signal ) ( formula 1 ). ( distribution value of shot noise in jitter component of receiver side )× α2 & lt ;( minimum of difference between received multi - value signal and logic threshold setting value ) ( formula 2 ). ( maximum of amplitude change in transmitting multi - value signal )× α4 & lt ;( limit value of amplitude in transmitter side optical signal ) ( formula 3 ). formula 1 expresses a condition for embodying the encrypted data transmission where the step in the multi - value signal is set smaller than ( or a half of ) the distribution value of shot noise . this provides every eavesdropper , who cannot obtain the setting of the logic threshold , with an eavesdropped noisy condition ( where the signal can hardly be received without errors ). formula 2 expresses a condition for allowing the proper receivers to receive the signal without error where the minimum or lower limit of a difference between the multi - value signal and the logic threshold setting value is greater than the distribution value of shot noise . this allows the authorized receiver , who has obtained the logic threshold , to detect data from the multi - value transmission signal without error . fig9 illustrates an example of the relation in the intensity between the signals . formula 3 defines the amplitude of a laser signal of the transmitter side . it is essential that the amplitude of the encrypted data signal to be transmitted is smaller than the performance of laser defined in fig1 . fig1 illustrates specification method of the performance of the laser in the transmitter side . when the unit period assumed for the transmission is t1 , the optical signal has to vary the intensity within to ( 10 % of t1 ). it is assumed from the laser to be used can output and modify the varying range of the optical signal . the rise of the signal in the duration t0 is vup and the decay is vdown . in the two values , the vup and the vdown , the smaller is defined as amplitude limit value of the optical signal in the transmitter side expressed by formula 3 . when the encrypted data transmission of the first embodiment is carried out under the conditions denoted by formulas 1 , 2 , and 3 , it can permit the proper receivers to receive the correct multi - value signal under a favorable condition while providing any eavesdroppers with an eavesdropped noisy condition . the data transmission apparatus of the first embodiment has three separate optical transmission systems to realize the signal transmission using fiber , respectively . in the embodiment shown in fig1 , the communication structure using three fibers are replaced by a combination of a wave mixer connected to the rear end of an optical transmitter equal to that of the first embodiment and a wave separator connected to the end of an optical receiver equal to that of the first embodiment . more specifically , the three laser emitters included in the optical transmitter of the encryption optical signal transmitter , the data transmitter 1 , and the data transmitter 2 are set to different wavelengths . this allows both the encryption communication and the two - value digital communication system to be carried out over a single fiber in a known multiple wavelength communication technique . since this embodiment allows the overall transmission system to be structured with a single optic fiber , it can significantly be lower in the cost of fiber installation than the first embodiment particularly in the case of long distance transmission where the cost of fiber installation is crucial . while the transmission apparatus of the first embodiment is based on one of encryption transmitter and receiver and two systems of two - value digital transmission systems , this embodiment comprises a pair of encryption signal transmitter and receiver and two systems of two - value digital transmitter system as shown in fig1 . two optical switches 1 and 2 are provided in the transmitter end and the receiver end respectively for switching between the encryption transmission and the two - value data transmission . each of the optical switches 1 and 2 has a 2 : 1 port arrangement for not conducting the encryption transmission and the two - value data transmission at the same time . as compared with the foregoing multiple wavelength system , the apparatus of this embodiment needs not to vary the wavelength of laser between the optical encrypted signal transmitter and the data transmitter . also , as this embodiment employs none of the expensive wave mixer and separator , its system can further be reduced in the overall cost .