Patent Publication Number: US-2022218280-A1

Title: Biological information monitoring system

Description:
RELATED APPLICATIONS 
     This application is a Continuation-in-Part (CIP) of U.S. patent application Ser. No. 17/131,761 filed on Dec. 23, 2020, which claims the benefit of priority of Japanese Patent Application No. 2020-166699 filed on Oct. 1, 2020. The contents of the above applications are all incorporated by reference as if fully set forth herein in their entirety. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     (i) Technical Field 
     The present disclosure relates to a biological information monitoring system. 
     (ii) Related Art 
     A technology for monitoring biological information, such as brain waves, is known. 
     Japanese Unexamined Patent Application Publication No. 2011-217986 discloses a technology for monitoring brain waves with a canal-type earphone including a conductive member. 
     SUMMARY OF THE INVENTION 
     Aspects of non-limiting embodiments of the present disclosure relate to making it possible to charge a biological information monitoring apparatus while reducing the influence on biological information to be monitored. 
     Aspects of certain non-limiting embodiments of the present disclosure overcome the above disadvantages and/or other disadvantages not described above. However, aspects of the non-limiting embodiments are not required to overcome the disadvantages described above, and aspects of the non-limiting embodiments of the present disclosure may not overcome any of the disadvantages described above. 
     According to an aspect of the present disclosure, there is provided a biological information monitoring system including: a biological information monitoring apparatus that monitors biological information of an organism wearing the biological information monitoring apparatus on an ear; and a charger that charges the biological information monitoring apparatus in a non-contact manner. The charger charges the biological information monitoring apparatus when the biological information monitoring apparatus is not monitoring biological information. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       An exemplary embodiment of the present disclosure will be described in detail based on the following figures, wherein: 
         FIGS. 1 ( 1 ) and  1 ( 2 ) are block diagrams of a biological information monitoring system according to the exemplary embodiment; 
         FIG. 2  is a perspective view illustrating the overall configuration of a biological information monitoring apparatus; 
         FIG. 3  illustrates the biological information monitoring apparatus as viewed from above; 
         FIG. 4  illustrates a left-side earphone as viewed from the left side; 
         FIG. 5  is a perspective view of a right-side earphone; 
         FIG. 6  is a perspective view of the left-side earphone; 
         FIG. 7  illustrates the right-side earphone as viewed from above; 
         FIG. 8  is an exploded perspective view of the right-side earphone; 
         FIG. 9  is a sectional view of the right-side earphone; 
         FIG. 10  is a perspective view illustrating the inside of the left-side earphone; 
         FIGS. 11 and 12  are perspective views of the left-side earphone; 
         FIGS. 13 and 14  are exploded perspective views of the left-side earphone; 
         FIG. 15  is a perspective view illustrating a main substrate and a sub-substrate; 
         FIG. 16  illustrates the inner surface of the main substrate; 
         FIG. 17  illustrates the outer surface of the main substrate; 
         FIG. 18  illustrates the inner surface of the sub-substrate; 
         FIG. 19  illustrates the outer surface of the sub-substrate; 
         FIG. 20  illustrates wiring on the inner surface of the main substrate; 
         FIG. 21  illustrates substrate parts included in the main substrate; 
         FIG. 22  illustrates wiring on the front surface of a relay board; 
         FIG. 23  illustrates wiring on the back surface of the relay board; 
         FIG. 24  is a block diagram illustrating functions of the biological information monitoring apparatus; 
         FIG. 25  is a block diagram illustrating functions of a terminal apparatus; 
         FIG. 26  schematically illustrates the external appearance of a right ear of a human; 
         FIG. 27  schematically illustrates the external appearance and the inside of a right ear of a human; 
         FIG. 28  schematically illustrates the external appearance of a right ear of a human; 
         FIGS. 29, 30, and 31  schematically illustrate a right-side earpiece and a conductive tube member; 
         FIG. 32  is a sectional view illustrating another example of the right-side earphone; 
         FIG. 33  illustrates the external appearance of a right-side earphone having a flexible structure; 
         FIG. 34  illustrates the external appearance of a left-side earphone according to a first modified example; 
         FIG. 35  illustrates the external appearance of a left ear of a human; 
         FIG. 36  illustrates the external appearance of a left-side earphone, which is an alternative to the first modified example; 
         FIG. 37  illustrates the external appearance of a left ear of a human; 
         FIG. 38  illustrates a left-side earphone according to a second modified example as viewed from the left side; 
         FIG. 39  illustrates the left-side earphone according to the second modified example as viewed from above; 
         FIG. 40  illustrates the external appearance of a left ear of a human; 
         FIG. 41  illustrates a right-side earphone according to a third modified example as viewed from the right side; 
         FIG. 42A  is a perspective view illustrating a microneedle sheet; 
         FIG. 42B  is a side view schematically illustrating a microneedle sheet; 
         FIG. 42C  is a schematic view illustrating a microneedle sheet as viewed from above; 
         FIG. 43  is a perspective view illustrating a microneedle sheet; 
         FIG. 44A  is a sectional view schematically illustrating an ear canal; 
         FIG. 44B  is a sectional view of a microneedle sheet and an elastic base layer; 
         FIG. 45  is a sectional view schematically illustrating an ear canal; 
         FIG. 46  is a perspective view of an induced pluripotent stem (iPS) cell sheet; 
         FIGS. 47, 48, and 49  illustrate the head of a human and a cushion as viewed from above; 
         FIGS. 50 and 51  illustrate the head of a human and a display as viewed from above; 
         FIG. 52  is a sectional view illustrating a cable of a first example; 
         FIG. 53  is a sectional view illustrating a cable of a second example; 
         FIG. 54  is a sectional view illustrating a cable of a third example; 
         FIG. 55  is a sectional view illustrating a cable of a fourth example; 
         FIG. 56  is a sectional view illustrating a cable of a fifth example; 
         FIG. 57  is a sectional view illustrating a cable of a sixth example; 
         FIG. 58  is a sectional view illustrating a cable of a seventh example; 
         FIG. 59  is a schematic view illustrating a cable of an eighth example, a right-side earphone, and a left-side earphone; 
         FIG. 60  illustrates a balanced differential amplifier; 
         FIG. 61  illustrates an approach to selecting electrodes according to a first example; 
         FIG. 62  illustrates an approach to selecting electrodes according to a second example; 
         FIG. 63  illustrates an approach to selecting electrodes according to a third example; and 
         FIG. 64  is a block diagram illustrating the configuration of a device for processing multiple channels of biological information. 
         FIG. 65  illustrates an experimental setup in which research electroencephalometer is worn together with a biological information monitoring apparatus; 
         FIG. 66  illustrates a graph illustrating the frequency characteristic from the experiment of  FIG. 65 ; 
         FIGS. 67 ( 1 ),  67 ( 2 ),  67 ( 3 ) and  67 ( 4 ) illustrate input-output characteristics of the biological monitoring apparatus; 
         FIG. 68  illustrates a result of the noise-evaluation of the biological monitoring apparatus near 0V; 
         FIG. 69  illustrates a result of the noise-evaluation of the result of  FIG. 68  according to JIS standard; 
         FIG. 70  illustrates a result of the noise-evaluation with application of a low-frequency cut-off filter. 
         FIG. 71  illustrates a configuration of left/right ear accessory. 
         FIG. 72  illustrates a biological information monitoring apparatus in the form of a wireless ear accessory; 
         FIGS. 73 ( 1 ) and  73 ( 2 ) illustrate alternate embodiments of an ear accessory; 
         FIG. 74  illustrates a configuration of an ear-hanger; 
         FIG. 75  illustrates an example positioning and attachment of electrodes; 
         FIG. 76  illustrates an earpiece with two independent mushroom-shaped electrodes; 
         FIG. 77  illustrates an example of a multi-flange earpiece; 
         FIG. 78  illustrates an example placement of the first electrode and the second electrode on the earpiece axis; 
         FIGS. 79 ( 1 ) and  79 ( 2 ) illustrate alternate embodiments of the earpiece; and 
         FIG. 80  illustrates an anatomy of the ear in relation to the brain and the jaw. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION 
     A biological information monitoring system  10  according to an exemplary embodiment will be described below with reference to  FIG. 1  ( 1 ).  FIG. 1  ( 1 ) illustrates an example of the configuration of the biological information monitoring system  10 . The biological information monitoring system  10  includes a biological information monitoring apparatus  12  and a terminal apparatus  14 . 
     The biological information monitoring apparatus  12  and the terminal apparatus  14  each have a function of communicating with another apparatus via a communication channel by using a wired medium, such as a cable, or a wireless medium. That is, the biological information monitoring apparatus  12  and the terminal apparatus  14  may be physically connected to another apparatus via a cable and send and receive information with this apparatus, or may send and receive information with another apparatus by wireless communication. Examples of wireless communication are short-range wireless communication, Wi-Fi (registered trademark), infrared communication, and visible light communication. Another wireless communication standard may be employed. Examples of short-range wireless communication are Bluetooth (registered trademark), radio frequency identifier (RFID), and near field communication (NFC). The biological information monitoring apparatus  12  and the terminal apparatus  14  may alternatively communicate with another apparatus, such as a server, via a communication channel, such as a local area network (LAN) or the Internet. The biological information monitoring apparatus  12  may also operate in coordination with an external sound input/output device  13  as shown in  FIG. 1  ( 2 ). For example, the biological information monitoring apparatus  12  and the sound input/output device  13  may communicate with each other wirelessly and operate by mutual sharing of data. The example of such a configuration with the biological information monitoring apparatus  12  being an ear accessory is shown in  FIG. 73  ( 2 ). 
     The biological information monitoring apparatus  12  monitors biological information concerning organisms, such as humans, animals other than humans, and plants. 
     Biological information include various types of information generated from organisms. The concept of biological information covers information concerning brain activities (such as brain waves, brain blood flow, and brain magnetic field signal), pulse rate, myoelectric information, such as myoelectric waveforms, saliva (such as the amount of saliva), pulse waves, blood pressure, blood flow, pulse, heart rate, electrocardiogram waveforms, eye movement, body temperature, amount of perspiration, gaze, voice, motion of an organism, and body fluids, such as blood. Information obtained by a biomarker may be used as biological information. Biological information may be information indicating potentials generated from an organism. For example, biological information may be brain waves, such as electroencephalograms (EEG), obtained by measuring minute electric currents generated by the brain activities, electrocardiograms created by measuring minute electric currents generated by the heart pulsating beats, electromyograms created by measuring minute electric currents generated by the muscle activities, and skin potentials obtained by measuring minute electric currents generated in the skin. The above-described items of information are only examples of biological information, and other items of biological information may be used. The biological information monitoring apparatus  12  monitors one or multiple types of biological information. 
     As a result of analyzing biological information, mental, emotional, or psychological information may be obtained. For example, as a result of analyzing biological information about a certain person, information indicating the mental state, the emotional state, or the psychological state of this person may be obtained. Biological information concerning an animal other than a human or a plant may be analyzed to obtain information indicating the state of the animal or the plant. 
     The biological information monitoring apparatus  12  may be a wearable device which can be worn by an organism and monitors biological information of the organism. The biological information monitoring apparatus  12  may be a device which monitors biological information of the organism without being worn by the organism. 
     Examples of a wearable device are a hearable device worn on an ear of an animal, a device worn on the head of an animal, a device worn around the wrist, arm, or finger of an animal (for example, a watch-type device, such as a smart watch), a device worn around the neck of an animal, a device worn on the torso of an animal (the abdomen or chest, for example), a device worn on the lower limb (the thigh, lower leg, knee, foot, or ankle of a human, for example), a pair-of-glasses-type device, and a contact-lens-type device. The biological information monitoring apparatus  12  may be a wearable device worn by a part of the body other than the above-described parts. The biological information monitoring apparatus  12  may be worn by multiple parts of the body. 
     A hearable device may be an earphone, a hearing aid, an earring-type device (a pierced-earring type device), an ear attachment accessories, ear-buds, an ear-wear, a clip-type device (an ear-clip type device), or a device with a band or a cable wound around the ear. A device worn on the head may be a headset with a band or a cable wound around the head. A device worn around the wrist, arm, or finger may be a device with a band or a cable wound around the wrist, arm, or finger. A device worn by another part of the body may have a band or a cable. 
     The biological information monitoring apparatus  12  may be a contact-type device that is brought into contact with an organism to monitor biological information of the organism or a non-contact-type device that is not brought into contact with an organism to monitor biological information of the organism. The biological information monitoring apparatus  12  may have both the functions as a contact-type device and a non-contact-type device. That is, the biological information monitoring apparatus  12  may monitor biological information in contact with an organism and also monitor biological information without contacting an organism. Biological information monitored by the contact-type device and that by the non-contact-type device may be the same type of biological information or different types of biological information. 
     The biological information monitoring apparatus  12  includes an electrode and a sensor for monitoring biological information. The electrode may detect potentials, which are an example of biological information, in contact with or without contacting an organism. The sensor may monitor biological information in contact with or without contacting an organism. For example, the electrode may be brought into contact with an animal to detect potentials representing brain waves of the animal. The sensor may monitor biological information indicating the body temperature of an animal without contacting it. These are only example of biological information, and other biological information may be monitored. 
     For example, the biological information monitoring apparatus  12  includes one or multiple electrodes. Multiple electrodes may be applied to an organism so as to detect potentials of the organism. The detected potentials include bioelectric potentials, which are an example of biological information of this organism. As a result of processing the detected potentials, the bioelectric potentials can be extracted from the detected potentials. For example, the detected potentials may include noise which does not originate from the bioelectric potentials of an organism, and as a result of executing processing, such as noise cancelling, the bioelectric potentials free from noise can be obtained. Potentials that can be identified as noise are potentials generated by the motion of an organism, potentials transmitted from the outside of an organism, potentials generated due to global environments, potentials representing biological information other than biological information to be monitored. Potentials generated from devices, such as a personal computer (PC) and a smartphone, may be an example of noise. Among the multiple electrodes, the electrode for detecting potentials may be switched in accordance with the detection sensitivity or the monitoring situation, such as the occurrence of noise. If various types of bioelectric potentials are detected together, they may be separated into individual types of potentials from each other according to the frequency. Alternatively, on the monitoring time axis, mixed different types of bioelectric potentials are divided into portions strongly related to each other and portions only sparsely related to each other, thereby separating the mixed bioelectric potentials into individual types of potentials. For example, in the above-described hearable device, information indicating the brain activities, information indicating the pulse rate, myoelectric information originating from the motion of muscles, and information originating from the blood flow, such as the pulse and the heart rate, may be monitored in a mixed manner. For these different types of information, the frequency or the absolute value of the output level is usually different. As a result of conducting the frequency analysis on a monitoring signal or based on the difference in the level of the output value of the monitoring signal, these types of information can be separated from each other. For example, brain waves can be separated into α waves, β waves, θ waves, δ waves, and γ waves according to the frequency. A different approach may be taken. For example, biological information is monitored in a hearable device under certain conditions. Then, under the same conditions, commercially available measuring devices, each of which is specially used for a specific type of biological information, such as an electroencephalograph, an electromyograph, and a blood flow measuring device, are used to monitor the corresponding types of biological information multiple times. 
     Then, a monitoring signal representing multiple types of biological information obtained by the hearable device is analyzed by using a frequency analysis technique, such as Fourier transform or wavelet transform, and then, statistical processing is also executed on the analysis result and monitoring signals obtained by the individual measuring devices. As a result, the monitoring signal can be separated into individual types of biological information. 
     In one example, one or plural electrodes are used as electrodes for detecting potentials including bioelectric potentials (such an electrode will be called a sensor electrode). Another one or plural electrodes are used as electrodes for grounding (such an electrode will be called a ground electrode). Another one or plural electrodes are used as electrodes for detecting potentials to be compared with potentials detected by the sensor electrodes (such an electrode will be called a reference electrode). 
     Hereinafter, potentials detected by the sensor electrodes will be called sensor potentials, potentials detected by the reference electrodes will be called reference potentials, and potentials detected by the ground electrodes will be called ground potentials. 
     In another example, the biological information monitoring apparatus  12  includes one or multiple sensors. For example, multiple sensors are applied to an organism so as to monitor biological information of the organism. The monitored biological information may contain noise. Among the multiple sensors, the sensor to be used for monitoring biological information may be switched in accordance with the detection sensitivity or the monitoring situation, such as the occurrence of noise. 
     In another example, the biological information monitoring apparatus  12  may include one or multiple electrodes and one or multiple sensors. 
     Plural biological information monitoring apparatuses  12 , each of which includes at least one of at least one electrode and at least one sensor, may be used, and may monitor the same type or different types of biological information. 
     The biological information monitoring apparatus  12  may send biological information, such as a potential signal indicating potentials detected by the electrode, information monitored by the sensor, and analysis information, to the terminal apparatus  14 . The biological information monitoring apparatus  12  may send biological information to a device, such as a server, other than the terminal apparatus  14 . In addition to or instead of sending biological information to another device, the biological information monitoring apparatus  12  may store the biological information therein. 
     The terminal apparatus  14  is a PC, a mobile terminal (such as a smartphone, a tablet PC, or a cellular phone), or another device, such as a music player. The terminal apparatus  14  may receive biological information from the biological information monitoring apparatus  12  and store, analyze, or send the received biological information to another device. For example, the terminal apparatus  14  may extract biological information from the potentials detected from an organism. 
     In one example, the biological information monitoring apparatus  12  is a bearable device, which is used by being inserted into the ear canal of a human. The biological information monitoring apparatus  12  may be a device including a canal-type earphone. As a function of the earphone, the biological information monitoring apparatus  12  converts an electric signal output from a playback device into sound waves by using a speaker. The playback device may be the terminal apparatus  14 . For example, the biological information monitoring apparatus  12  receives a sound signal, such as an audio signal, from the terminal apparatus  14  by wired or wireless communication and generates sound in accordance with the sound signal. 
     The configuration of the biological information monitoring apparatus  12  will be described below in detail with reference to  FIGS. 2 through 10 . The biological information monitoring apparatus  12  will be discussed through illustration of earphones worn on the ears of a human. However, it may be a bearable device other than earphones. 
       FIG. 2  is a perspective view illustrating the overall configuration of the biological information monitoring apparatus  12 .  FIG. 3  illustrates the biological information monitoring apparatus  12  as viewed from above.  FIG. 4  illustrates a left-side earphone as viewed from the left side.  FIG. 5  is a perspective view of a right-side earphone.  FIG. 6  is a perspective view of the left-side earphone.  FIG. 7  illustrates the right-side earphone as viewed from above.  FIG. 8  is an exploded perspective view of the right-side earphone.  FIG. 9  is a sectional view of the right-side earphone.  FIG. 10  is a perspective view illustrating the inside of the left-side earphone. 
     For the sake of description, the front and back directions, left and right directions, and top and bottom directions are defined, as shown in  FIG. 2 . The front direction is a direction in which a user faces in front. The back direction is a direction opposite the front direction. The top direction is a direction in which the top of the head of a user faces. The bottom direction is a direction opposite the top direction. The right direction is a direction on the side of the right hand of a user, while the left direction is a direction on the side of the left hand of a user. The front-back direction, the top-bottom direction, and the left-right direction are perpendicular to each other. 
     In one example, the biological information monitoring apparatus  12  monitors biological information including information indicating brain waves. The biological information monitoring apparatus  12  may monitor another type of biological information in addition to brain waves. For example, when a right-side earphone  16 R and a left-side earphone  16 L, which will be explained later, are worn on the ears of a user, potentials representing biological information are detected. A potential signal indicating the detected potentials typically represents brain waves, but may also represent potentials generated from the motion of the head, such as the motion of the face due to facial expressions, and the motion of the neck, jaws, and eyeballs. Additionally, biological information generated by a change in the blood flow caused by the motion of the head, such as the pulse waves related to the brain blood flow and the heart rate related to the cardiac blood vessels, may distinctively be monitored. In this manner, the biological information monitoring apparatus  12  may monitor other types of biological information, together with brain waves. Biological information other than brain waves may be handled as noise (eliminated, for example), or be separated from the brain waves and be used for certain processing, as described above. 
     As shown in  FIGS. 2 and 3 , the biological information monitoring apparatus  12  is largely constituted by a right-side earphone  16 R to be worn on the right ear of a user, a left-side earphone  16 L to be worn on the left ear of a user, and a cable  18  electrically connecting the right-side earphone  16 R and the left-side earphone  16 L with each other. The right-side earphone  16 R and the left-side earphone  16 L may send and receive signals and data with each other via the cable  18 . A remote controller may be provided in the cable  18  to operate the biological information monitoring apparatus  12 . 
     One or both of the right-side earphone  16 R and the left-side earphone  16 L function as the biological information monitoring apparatus  12  that monitors biological information. 
     For example, one of the right-side earphone  16 R and the left-side earphone  16 L may have a sensor electrode, a reference electrode, and a ground electrode, and the other earphone may have no electrodes. 
     In another example, each of the right-side earphone  16 R and the left-side earphone  16 L may have a sensor electrode, a reference electrode, and a ground electrode. 
     In still another example, one of the right-side earphone  16 R and the left-side earphone  16 L may have one or two electrodes among a sensor electrode, a reference electrode, and a ground electrode, while the other earphone may have the remaining electrode or electrodes. 
     In another example, each of the right-side earphone  16 R and the left-side earphone  16 L may have one or plural electrodes, and such electrodes may be allocated to a sensor electrode, a reference electrode, and a ground electrode. 
     In another example, each of the right-side earphone  16 R and the left-side earphone  16 L may have plural sensor electrodes, plural reference electrodes, or plural ground electrodes. For example, the right-side earphone  16 R may have plural sensor electrodes, while the left-side earphone  16 L may have plural reference electrodes. 
     In another example, the earphones may be made wireless without the use of cable  18 . Each earphone may have a sensor electrode, a reference electrode and a ground electrode with the left-side earphone  16 L and the right-side earphone  16 R being able to monitor independently. This type of example will be discussed in more detail with respect to  FIGS. 71-74  below. 
     The right-side earphone  16 R is a canal-type earphone, for example, and includes a right-side housing  20 R, a right-side earpiece  22 R, a right-side support  24 R, a right-side ear hanger  26 R, and an electrode member  28 R. The earpiece may also be called an earpad. 
     The right-side housing  20 R is a thin rectangular-parallelopiped-shaped housing which stores elements such as an electronic circuit therein. In the right-side housing  20 R, the right-side earpiece  22 R and the right-side support  24 R are located facing the right ear of a user when the user wears the biological information monitoring apparatus  12 . Within the right-side housing  20 R, a controller, a speaker, a communication unit (communication chip, for example), an electronic circuit for analyzing and processing biological information, and a 6-axis sensor (including a 3-axis sensor for detecting the acceleration and a 3-axis sensor for detecting the angular velocity, for example), a memory, a global positioning system (GPS) sensor, for example, are stored. The communication unit implements a wireless communication function, such as Bluetooth or Bluetooth Low Energy (BLE). The communication unit loads a wireless LAN (such as a Wi-Fi network) and a cellular (3G, 4G, 5G, or low-power wide-area (LPWA)) module to achieve a wider communication area and can thus directly send data to local area devices having a communication range longer than Bluetooth and to remote devices via the Internet. Using the 6-axis sensor makes it possible to detect the moving direction, orientation, and rotation of the right-side housing  20 R. Biological information may be stored in the memory. The electronic circuit for analyzing biological information may not necessarily be provided in the biological information monitoring apparatus  12 . 
     The right-side support  24 R has a columnar shape, such as a cylindrical shape, and projects from the side surface of the right-side housing  20 R (for example, the side surface which opposes the right ear when the right-side earphone  16 R is worn on the right ear of a user). The right-side support  24 R is disposed between the right-side housing  20 R and the right-side earpiece  22 R. The outer diameter of the right-side support  24 R is larger than that of the right-side earpiece  22 R. The electrode member  28 R is provided on the entirety or part of the side surface of the right-side support  24 R. 
     The electrode member  28 R has a ring-like shape, for example, and is supported by the columnar right-side support  24 R. The entirety or part of the electrode member  28 R serves as an electrode. That is, an electrode may be provided on the entire surface or part of the surface of the electrode member  28 R. The electrode member  28 R is made of conductive carbon rubber, for example. In this case, too, the electrode member  28 R may be entirely made of conductive carbon rubber and the entirety (entire surface, for example) of the electrode member  28 R may serve as an electrode. Alternatively, the electrode member  28 R may be partially made of conductive carbon rubber and part (part of the surface, for example) of the electrode member  28 R may serve as an electrode. 
     The right-side earpiece  22 R is provided at the end of the right-side support  24 R. More specifically, the right-side earpiece  22 R is provided at the end of the right-side support  24 R opposite the end contacting the right-side housing  20 R. The right-side earpiece  22 R has a cylindrical shape tapering down toward the forward end, but may be formed in another shape. 
     Within the right-side earpiece  22 R, a sound transmission tube (such as a conductive tube member  30 R, which will be discussed later), for example, is stored. Sound emitted from a speaker is transmitted through the sound transmission tube within the right-side earpiece  22 R and is output from the right-side earpiece  22 R to the outside. An electrode is provided on the entirety or part of the outer surface (side surface, for example) of the right-side earpiece  22 R. The electrode is made of conductive carbon rubber, for example. The right-side earpiece  22 R itself may be made of conductive rubber. For example, the entirety or part of the right-side earpiece  22 R may be made of conductive rubber. That is, the entirety or part of a surface of the right-side earpiece  22 R may serve as an electrode. 
     The right-side earpiece  22 R and the electrode member  28 R may be made of an elastic member. As the elastic member, a resin, such as rubber, may be used. More specifically, silicon (Si) rubber (S1734 made by NOK CORPORATION) or urethane rubber may be used for the right-side earpiece  22 R and the electrode member  28 R. The hardness of the right-side earpiece  22 R and that of the electrode member  28 R in compliance with the standard of durometer type A (instantaneous) are 40 to 75. For example, a resin having a hardness of 70 may be used for the right-side earpiece  22 R and the electrode member  28 R. 
     The right-side earpiece  22 R is inserted into and is placed in the canal of the right ear of a user so as to contact the ear canal. The electrode member  28 R supported by the right-side support  24 R is placed on the right ear so as to contact the cavity of concha. This will be discussed in detail later. 
     The right-side ear hanger  26 R is generally formed in a curved shape. When a user wears the right-side earphone  16 R, the right-side ear hanger  26 R is hung on the right ear. For example, the right-side ear hanger  26 R is hung on the helix of the right ear. More specifically, the right-side ear hanger  26 R is placed on the back side of the helix of the right ear so as to contact the helix. One end of the right-side ear hanger  26 R is connected to the front side of the right-side housing  20 R and is curved toward the back side of the right-side housing  20 R. When the user wears the right-side earphone  16 R, this curved portion is placed on the back side of the helix of the right ear so as to touch the helix. For example, the curved portion is formed to match the shape of the back side of the helix of the right ear. The other end of the right-side ear hanger  26 R is connected to one end of the cable  18 . 
     The right-side earpiece  22 R and the right-side support  24 R are replaceable. For example, multiple (three to five) types of right-side earpieces  22 R and those of right-side supports  24 R of different shapes and different sizes are prepared, and a suitable right-side earpiece  22 R and a suitable right-side support  24 R are selected in accordance with the shape of the right ear (such as the shapes of the ear canal, cavity of concha, and other parts) of a user. 
     The left-side earphone  16 L is a canal-type earphone, for example, and includes a left-side housing  20 L, a left-side earpiece  22 L, a left-side support  24 L, a left-side ear hanger  26 L, and an electrode member  28 L. 
     The left-side housing  20 L is a thin rectangular-parallelopiped-shaped housing which stores elements such as a battery therein. In the left-side housing  20 L, the left-side earpiece  22 L and the left-side support  24 L are located facing the left ear of a user when the user wears the biological information monitoring apparatus  12 . Power from the battery is supplied to the right-side earphone  16 R and the left-side earphone  16 L so as to drive them. Power from the battery is supplied to a speaker and individual circuits, for example. The battery may be provided in one or both of the right-side housing  20 R and the left-side housing  20 L. 
     The left-side support  24 L has a columnar shape, such as a cylindrical shape, and projects from the side surface of the left-side housing  20 L (for example, the side surface which opposes the left ear when the left-side earphone  16 L is worn on the left ear of a user). The left-side support  24 L is disposed between the left-side housing  20 L and the left-side earpiece  22 L. The outer diameter of the left-side support  24 L is larger than that of the left-side earpiece  22 L. The electrode member  28 L is provided on the entirety or part of the side surface of the left-side support  24 L. 
     The electrode member  28 L has a ring-like shape, for example, and is supported by the columnar left-side support  24 L. The entirety or part of the electrode member  28 L serves as an electrode. That is, an electrode may be provided on the entire surface or part of the surface of the electrode member  28 L. The electrode member  28 L is made of conductive carbon rubber, for example. In this case, too, the electrode member  28 L may be entirely made of conductive carbon rubber and the entirety (entire surface, for example) of the electrode member  28 L may serve as an electrode. Alternatively, the electrode member  28 L may be partially made of conductive carbon rubber and part (part of the surface, for example) of the electrode member  28 L may serve as an electrode. 
     The left-side earpiece  22 L is provided at the end of the left-side support  24 L. More specifically, the left-side earpiece  22 L is provided at the end of the left-side support  24 L opposite the end contacting the left-side housing  20 L. The left-side earpiece  22 L has a cylindrical shape tapering down toward the forward end, but may be formed in another shape. 
     Within the left-side earpiece  22 L, a sound transmission tube, for example, is stored. Sound emitted from the speaker is transmitted through the sound transmission tube within the left-side earpiece  22 L and is output from the left-side earpiece  22 L to the outside. An electrode is provided on the entirety or part of the outer surface (side surface, for example) of the left-side earpiece  22 L. The electrode is made of conductive carbon rubber, for example. The left-side earpiece  22 L itself may be made of conductive carbon rubber. For example, the entirety or part of the left-side earpiece  22 L may be made of conductive rubber. That is, the entirety or part of a surface of the left-side earpiece  22 L may serve as an electrode. 
     The left-side earpiece  22 L and the electrode member  28 L may be made of an elastic member. As the elastic member, a resin, such as rubber, may be used. More specifically, Si rubber (S1734 made by NOK CORPORATION) or urethane rubber may be used for the left-side earpiece  22 L and the electrode member  28 L. The hardness of the left-side earpiece  22 L and that of the electrode member  28 L in compliance with the standard of durometer type A (instantaneous) are 40 to 75. For example, a resin having a hardness of 70 may be used for the left-side earpiece  22 L and the electrode member  28 L. 
     The left-side earpiece  22 L is inserted into and is placed in the ear canal of the left ear of a user so as to contact the ear canal. The electrode member  28 L supported by the left-side support  24 L is placed on the left ear so as to contact the cavity of concha. This will be discussed in detail later. 
     The left-side ear hanger  26 L generally has a curved shape. When a user wears the left-side earphone  16 L, the left-side ear hanger  26 L is hung on the left ear. For example, the left-side ear hanger  26 L is hung on the helix of the left ear. More specifically, the left-side ear hanger  26 L is placed on the back side of the helix of the left ear so as to contact the helix. One end of the left-side ear hanger  26 L is connected to the front side of the left-side housing  20 L and is curved toward the back side of the left-side housing  20 L. When the user wears the left-side earphone  16 L, this curved portion is placed on the back side of the helix of the left ear so as to touch the helix. For example, the curved portion is formed to match the shape of the back side of the helix of the left ear. The other end of the left-side ear hanger  26 L is connected to one end of the cable  18 . 
     The left-side earpiece  22 L and the left-side support  24 L are replaceable. For example, multiple (three to five) types of left-side earpieces  22 L and those of left-side supports  24 L of different shapes and different sizes are prepared, and a suitable left-side earpiece  22 L and a suitable left-side support  24 L are selected in accordance with the shape of the left ear (such as the shapes of the ear canal, cavity of concha, and other parts) of a user. 
     The controller, the communication unit, the electronic circuit, the 6-axis sensor, the memory, and other elements provided in the right-side housing  20 R may be stored in one or both of the right-side housing  20 R and the left-side housing  20 L. 
     A power button and a volume control switch, for example, are provided in the right-side housing  20 R or the left-side housing  20 L. They may be provided both in the right-side housing  20 R and the left-side housing  20 L. 
     One of the electrode disposed in the right-side earpiece  22 R and that in the left-side earpiece  22 L is used as a sensor electrode, and the other one of the electrodes is used as a reference electrode. The electrode members  28 R and  28 L are used as ground electrodes. Alternatively, one of the electrode members  28 R and  28 L may be used as a sensor electrode, while the other one of the electrode members  28 R and  28 L may be used as a reference electrode, and the electrode provided in the right-side earpiece  22 R and that in the left-side earpiece  22 L may be used as ground electrodes. 
     In another example, multiple electrodes may be provided separately from each other in the right-side earpiece  22 R, and at least one of the multiple electrodes may be selected as a sensor electrode, a reference electrode, or a ground electrode to be used. For example, the multiple electrodes may be used as sensor electrodes, and among the sensor electrodes, the potential detected by the electrode having the highest detection sensitivity, the electrode having the smallest occurrence of noise, or the electrode having the stable level of noise may be used as the sensor potential. When the multiple electrodes provided in the right-side earpiece  22 R are used as reference electrodes or ground electrodes, the reference electrode or the ground electrode to be used may be selected similarly. Multiple electrodes may be provided in each of the left-side earpiece  22 L and the electrode members  28 R and  28 L, and the electrode to be used may be selected similarly. 
     As shown in  FIG. 7 , the electrode member  28 R supported by the right-side support  24 R is disposed at a position separated from the right-side housing  20 R by an amount of a height h. That is, the distance between the electrode member  28 R and the right-side housing  20 R is set to be the height h. The height h is set to such a degree as to prevent the interference between the right-side housing  20 R and the helix of the right ear and thus to avoid the occurrence of a contact failure. More specifically, the height h is set so that the right-side housing  20 R does not touch the helix of the right ear when the right-side earpiece  22 R is inserted into the canal of the right ear and the right-side earphone  16 R is worn on the right ear. That is, the height h is set so that the right-side housing  20 R is sufficiently separated from the helix of the right ear so as not to contact the helix. This can avoid the contact between the top surface of the right-side housing  20 R and the helix of the right ear. If the right-side housing  20 R touches and interferes with the helix of the right ear, the right-side earpiece  22 R may not be inserted into a position at which it can stably be placed in the ear canal, which may cause a contact failure between the electrode of the right-side earpiece  22 R and the ear canal. Moreover, the electrode member  28 R may not be located properly to contact the cavity of concha of the right ear, which may cause a contact failure between the electrode member  28 R and the cavity of concha. The occurrence of a contact failure decreases the detection sensitivity for the sensor potential, reference potential, and ground potential and accordingly reduces the accuracy in monitoring biological information. As a result of setting the height h to a suitable value so that the right-side housing  20 R does not touch the helix of the right ear, the interference between the right-side housing  20 R and the helix of the right ear can be avoided. The right-side earpiece  22 R can be inserted into a position at which it is stably located in the ear canal, thereby achieving a good contact between the electrode of the right-side earpiece  22 R and the ear canal. Setting the height h to a suitable value also makes it possible to dispose the electrode member  28 R properly to contact the cavity of concha of the right ear, thereby achieving a good contact between the electrode member  28 R and the cavity of concha. As a result, the detection sensitivity for bioelectric potentials is improved. The above-described height h is also set to be a suitable value in the left-side earphone  16 L. 
     As shown in  FIG. 3 , when the right-side earphone  16 R is viewed from above, the right-side ear hanger  26 R is displaced from the right-side housing  20 R in the direction in which the right-side earpiece  22 R is located. That is, in a plan view of the right-side earphone  16 R, a gap is formed between the right-side housing  20 R and the right-side ear hanger  26 R. The gap is a space for the right ear, and the right ear is fit in this space. The right ear can be inserted into the space regardless of its size, and the right-side earphone  16 R can be stably worn on the right ear. This can achieve a good contact between the electrode of the right-side earpiece  22 R and the ear canal and between the electrode member  28 R and the cavity of concha. The above-described gap is also provided between the left-side housing  20 L and the left-side ear hanger  26 L in the left-side earphone  16 L. 
     As shown in  FIG. 8 , a conductive tube member  30 R made of a metal is provided to protrude from the right-side support  24 R in the direction in which it is separated from the right-side housing  20 R. The rear end of the conductive tube member  30 R is placed at the right-side support  24 R, while the forward end is covered by the right-side earpiece  22 R. For example, the rear end of the conductive tube member  30 R is screwed into the forward end of the right-side support  24 R. A conductive tube member  32 R made of a metal is provided on the side surface of the right-side support  24 R in the direction in which it protrudes from this side surface. The ring-like electrode member  28 R is fit on the side surface of the right-side support  24 R to cover the conductive tube member  32 R. More specifically, a groove is formed on the side surface of the right-side support  24 R in the circumferential direction, and a hole  24 R 1  is formed in the groove to receive one end of the conductive tube member  32 R. This end of the conductive tube member  32 R is screwed into the hole  24 R 1 , so that the conductive tube member  32 R is fixed to the side surface of the right-side support  24 R. In the state in which the conductive tube member  32 R is fixed to the side surface of the right-side support  24 R, the electrode member  28 R is fit into the groove on the side surface of the right-side support  24 R. In this manner, the electrode member  28 R is fixed on the side surface of the right-side support  24 R while contacting the conductive tube member  32 R. The conductive tube member  32 R is connected to an electrical wire inside the right-side support  24 R. A potential signal indicating a potential detected by the electrode member  28 R is transmitted to the electrical wire via the conductive tube member  32 R and is then output to a main substrate  34 , which will be discussed later, via the electrical wire. The internal configuration of the left-side earphone  16 L is similar to that of the right-side earphone  16 R described above. 
     As shown in  FIG. 9 , the main substrate  34  and a sub-substrate  36  are stored in the right-side housing  20 R. Details of the main substrate  34  and the sub-substrate  36  will be discussed later. Substrates other than the main substrate  34  and the sub-substrate  36  may be stored in the left-side housing  20 L. 
     As shown in  FIG. 10 , a battery  38  and a relay board  40  are stored in the left-side housing  20 L. Power is supplied from the battery  38  to the right-side earphone  16 R and the left-side earphone  16 L via the relay board  40 . 
     The cable  18  is hard enough to maintain the overall shape of the cable  18 . For example, the cable  18  is formed in a shape in which it extends from the right-side housing  20 R and the left-side housing  20 L in the back direction so that the cable  18  does not touch the back of the head or the hair when the biological information monitoring apparatus  12  is worn on the ears of a user. This avoids the occurrence of noise in biological information, which would otherwise be produced as a result of the cable  18  touching the back of the head or the hair. 
     Even if the cable  18  touches the back of the head, for example, the resulting load is transmitted to the cable  18 , and the entirety of the left-side earphone  16 L is rotated in the back direction, as indicated by the arrow Y in  FIG. 4 . This can fix the left-side earpiece  22 L and the electrode member  28 L to the left ear more firmly. That is, the entirety of the left-side earphone  16 L is rotated in the direction in which the left-side earpiece  22 L and the electrode member  28 L are fixed more firmly. This achieves a good contact between the electrode of the left-side earpiece  22 L and the ear canal and between the electrode member  28 L and the cavity of concha, thereby enhancing the detection sensitivity. When the cable  18  touches the back of the head, the entirety of the right-side earphone  16 R is also rotated similarly to the left-side earphone  16 L. 
     The ear hangers will be explained below with reference to  FIGS. 11 through 14 .  FIGS. 11 and 12  are perspective views of the left-side earphone  16 L.  FIGS. 13 and 14  are exploded perspective views of the left-side earphone  16 L. 
     The left-side ear hanger  26 L includes a cover  42  that covers the cable  18  and a hanger member  44  that covers the cover  42 . The hanger member  44  is not shown in  FIGS. 11 and 13 . The cable  18  on which the left-side ear hanger  26 L is formed is covered with the cover  42 . The hanger member  44  is shown in  FIGS. 12 and 14 . The hanger member  44  is disposed to cover the cover  42 . 
     As shown in  FIGS. 13 and 14 , the left-side housing  20 L is constituted by housing members  46  and  48 . The left-side ear hanger  26 L is fixed to the left-side housing  20 L as a result of the forward end of the cover  42  (that is, the portion of the left-side ear hanger  26 L to be attached to the left-side housing  20 L) being sandwiched between the housing members  46  and  48 . The forward end of the hanger member  44 , as well as that of the cover  42 , may also be sandwiched between the housing members  46  and  48 . The right-side ear hanger  26 R is configured similarly to the left-side ear hanger  26 L. 
     The cover  42  is made of nylon, for example, and the hanger member  44  is made of silicon rubber, for example. Using nylon for the cover  42  can prevent the hanger member  44  from breaking off from its base which is fixed to the left-side housing  20 L. The cover  42  and the hanger member  44  can bend the cable  18  so that it can be prevented from touching a user when the user wears the biological information monitoring apparatus  12 . The right-side ear hanger  26 R is configured similarly to the left-side ear hanger  26 L. 
     The main substrate  34  and the sub-substrate  36  will be explained below.  FIG. 15  is a perspective view illustrating the positional relationship between the main substrate  34  and the sub-substrate  36 . A description will be given, assuming that biological information is brain waves and the main substrate  34  and the sub-substrate  36  include elements such as an electronic circuit for extracting brain waves from a potential signal indicating potentials detected by the electrodes. 
     The main substrate  34  has an inner surface  34   a  and an outer surface  34   b . The inner surface  34   a  opposes the head of a user wearing the right-side earphone  16 R. The outer surface  34   b  is the surface opposite the inner surface  34   a . The sub-substrate  36  has an inner surface  36   a  and an outer surface  36   b . The inner surface  36   a  opposes the head of a user wearing the right-side earphone  16 R. The outer surface  36   b  is the surface opposite the inner surface  36   a.    
       FIG. 16  illustrates the inner surface  34   a  of the main substrate  34 .  FIG. 17  illustrates the outer surface  34   b  of the main substrate  34 . 
     As shown in  FIG. 16 , a brain wave sensor  3410 , a 6-axis sensor  3411 , a flash memory  3412 , a light-emitting diode (LED)  3413 , a board-to-board connector  3414 , and a universal serial bus (USB) terminal  3415 , for example, are disposed on the inner surface  34   a  of the main substrate  34 . The brain wave sensor  3410  analyzes a potential signal indicating detected potentials so as to extract brain waves from the potential signal. 
     As shown in  FIG. 17 , a communication chip  3416  for performing communication based on Bluetooth, a charging control integrated circuit (IC)  3417  for controlling the charging of the battery, a buck-boost converter  3418 , a serial peripheral interface (SPI) memory  3419 , a static random access memory (SRAM)  3420 , a memory control unit (MCU)  3421 , and an antenna  3422  are disposed on the outer surface  34   b  of the main substrate  34 . 
       FIG. 18  illustrates the inner surface  36   a  of the sub-substrate  36 .  FIG. 19  illustrates the outer surface  36   b  of the sub-substrate  36 . 
     As shown in  FIG. 18 , a microphone  3610  is disposed on the inner surface  36   a  of the sub-substrate  36 . A through-hole  3611  is also formed to pass through the sub-substrate  36  in the thickness direction. The through-hole  3611  is a hole for wiring. 
     As shown in  FIG. 19 , volume control switches  3612  and  3613 , a switch  3614  for setting the functions of the biological information monitoring apparatus  12 , and a board-to-board connector  3615  are disposed on the outer surface  36   b  of the sub-substrate  36 . 
       FIG. 20  illustrates electrode pads on the inner surface  34   a  of the main substrate  34 . In  FIG. 20 , only the brain wave sensor  3410  and the electrode pads are shown and electrical wires on the main substrate  34  are not shown. 
     Electrode pads  34   a   1  and  34   a   2  are connected to electrodes provided in the right-side earphone  16 R via electrical wires. For example, the electrode pad  34   a   1  is connected to one of the electrode in the right-side earpiece  22 R and the electrode of the electrode member  28 R via an electrical wire, while the electrode pad  34   a   2  is connected to the other one of the electrodes via an electrical wire. The electrode pads  34   a   1  and  34   a   2  are connected to the brain wave sensor  3410  via electrical wires formed on the main substrate  34 . An electrode pad  34   a   3  is connected to a shielding ground electrode. The two electrical wires connected to the electrode pads  34   a   1  and  34   a   2  are bundled together by a shield line and extend to and are connected to the electrode provided in the right-side earpiece  22 R and to the electrode member  28 R via the through-hole  3611  formed in the sub-substrate  36 . 
     Electrode pads  34   a   4  and  34   a   5  are connected to the speaker provided in the right-side earphone  16 R via electrical wires. An electrode pad  34   a   6  is connected to a shielding ground electrode. The two electrical wires connected to the electrode pads  34   a   4  and  34   a   5  are bundled together by a shield line and are connected to the speaker. 
     Electrode pads  34   a   7  and  34   a   8  are connected to electrodes provided in the left-side earphone  16 L via electrical wires. For example, the electrode pad  34   a   7  is connected to one of the electrode in the left-side earpiece  22 L and the electrode of the electrode member  28 L via an electrical wire, while the electrode pad  34   a   8  is connected to the other one of the electrodes via an electrical wire. The electrode pads  34   a   7  and  34   a   8  are connected to the brain wave sensor  3410  via electrical wires formed on the main substrate  34 . An electrode pad  34   a   9  is connected to a shielding ground electrode. The two electrical wires connected to the electrode pads  34   a   7  and  34   a   8  are bundled together by a shield line and extend to and are connected to the electrode provided in the left-side earpiece  22 L and to the electrode member  28 L. 
     Electrode pads  34   a   10  and  34   a   11  are connected to the speaker provided in the left-side earphone  16 L via electrical wires. An electrode pad  34   a   12  is connected to a shielding ground electrode. The two electrical wires connected to the electrode pads  34   a   10  and  34   a   11  are bundled together by a shield line and are connected to the speaker. 
     Electrode pads (such as the electrode pads  34   a   1 ,  34   a   2 ,  34   a   7 , and  34   a   8 ) for monitoring brain waves are disposed closer to the brain wave sensor  3410  than the other electrode pads are. This can reduce the influence of noise occurring in wiring between the brain wave sensor  3410  and the electrode pads for monitoring brain waves. The communication chip  3416  and the charging control IC  3417  are not disposed between the brain wave sensor  3410  and the electrode pads for monitoring brain waves. This can reduce the influence of noise occurring in the communication chip  3416  and the charging control IC  3417 . 
     A group of the electrical wires connected to the electrode pads for monitoring brain waves and a group of the electrical wires used for the electrode pads connected to the speakers are laid separately from each other on the main substrate  34 . This can reduce the occurrence of crosstalk between the two groups of electrical wires. 
     The main substrate  34  may be a multilayer substrate.  FIG. 21  illustrates substrate parts  341  and  342  included in the main substrate  34 . The substrate parts  341  and  342  form different layers of the multilayer substrate. The brain wave sensor  3410  is provided in the substrate part  341 . The substrate part  342  is a shield ground (SGND) substrate and is overlaid on the substrate part  341 . Bioelectric potentials are input into the brain wave sensor  3410 . The substrate part  342 , which is an SGND substrate, is overlaid on the substrate part  341  in which the brain wave sensor  3410  is provided, thereby reducing noise occurring in bioelectric potentials. 
       FIG. 22  illustrates wiring on a front surface  40   a  of the relay board  40 .  FIG. 23  illustrates wiring on a back surface  40   b  of the relay board  40 . 
     An electrode pad  4000  is connected to the negative terminal of the battery  38  (see  FIG. 10 ), while an electrode pad  4001  is connected to the positive terminal of the battery  38 . 
     Electrode pads  4002  and  4003  are connected to the electrodes provided in the left-side earphone  16 L via electrical wires. For example, the electrode pad  4002  is connected to one of the electrode provided in the left-side earpiece  22 L or the electrode of the electrode member  28 L via an electrical wire, while the electrode pad  4003  is connected to the other one of the electrodes via an electrical wire. For example, the electrode pad  4002  is connected to the electrode provided in the left-side earpiece  22 L, while the electrode pad  4003  is connected to the electrode of the electrode member  28 L. An electrode pad  4004  is connected to a shielding ground electrode. 
     Electrode pads  4006  and  4007  are audio electrode pads and are connected to the speaker of the left-side earphone  16 L via electrical wires. An electrode pad  4005  is connected to a shielding ground electrode. 
     The electrode pads  4002  and  4003  are surrounded by a noise shields  4008  and  4010 , while the electrode pads  4006  and  4007  are surrounded by noise shields  4009  and  4011 . 
     A group of the electrode pads  4005  through  4007  used for audio signals and a group of the electrode pads  4002  through  4004  connected to the electrodes for detecting bioelectric potentials are disposed separately from each other, and are surrounded by the corresponding shields. This arrangement can reduce the occurrence of noise in bioelectric potentials caused by audio signals. 
     A group of the electrode pads  4000  and  4001  used for the battery  38  and a group of the electrode pads  4002  through  4004  are disposed separately from each other, and the electrode pads  4002  through  4004  are surrounded by the shields. This can reduce the occurrence of noise in bioelectric potentials caused by the battery  38 . 
     Functions of the biological information monitoring apparatus  12  will be described below in detail with reference to the block diagram of  FIG. 24 . 
     As described above, the biological information monitoring apparatus  12  includes the right-side earphone  16 R, the left-side earphone  16 L, and the cable  18 . The right-side earphone  16 R and the left-side earphone  16 L are physically connected with each other by the cable  18  and send and receive data with each other via the cable  18 . 
     The right-side earphone  16 R includes a right-side speaker  16 R 1 , a communication unit  16 R 2 , a first right-side electrode  16 R 3 , a second right-side electrode  16 R 4 , a processing unit  16 R 5 , a storage  16 R 6 , and a controller  16 R 7 . 
     The left-side earphone  16 L includes a left-side speaker  16 L 1 , a battery  16 L 2 , a first left-side electrode  16 L 3 , and a second left-side electrode  16 L 4 . 
     Sound emitted from the right-side speaker  16 R 1  is output from the right-side earpiece  22 R to the outside. Sound emitted from the left-side speaker  16 L 1  is output from the left-side earpiece  22 L to the outside. 
     As an example of an electrode used, an electrode that satisfies the JIS standard can be used. For instance, an input-output characteristic of an electrode may be pre-tested simultaneously with a medical electroencephalometer that is known to satisfy the JIS standard. 
     As shown in  FIG. 65 , an example test is conducted, in which the test subject simultaneously wears the biological information monitoring apparatus  12  and a research electroencephalometer that is known to satisfy the JIS standard, and their characteristics are compared. As shown in  FIG. 66 , a frequency analysis per second at 10 Hz, with two iterations of open-eye state and a close-eye state, shows a correlation of about 0.7. Therefore, the biological information monitoring apparatus  12  has been found to satisfy the JIS signal quality standard for medical electroencephalometer (telemetry electroencephalometer JIS T  1203 ). 
     Furthermore, a dummy brain-wave is input to the biological information monitoring apparatus  12 , and the linearity evaluation of the resulting input-output characteristics, with the applied voltage and the frequency as variables, is conducted. In this test, the electrodes are placed at the ear canal of the right ear, the ear canal of the left ear and the cavity of concha at the right ear. Three levels of applied frequencies are used, and applied voltage of 0V and eleven levels of applied voltages are used, and the output characteristics are obtained for 10 test iterations. The results are shown in  FIG. 67 . 
       FIG. 67  ( 1 ) illustrates that substantially linear characteristics are obtained in a range of 0-1000 μV.  FIG. 67  ( 2 ) illustrates that when a small input voltage is applied, the lower the frequency the more degradation in linearity is seen, and at 0V, an offset of about 2.5 μV occurs. For the ranges illustrated in  FIGS. 67  ( 3 ) and ( 4 ), substantially linear characteristics are obtained. 
     Based on the above result, a noise-evaluation of the input-output characteristics at or near 0V is conducted, and the result is shown in  FIG. 68 . Specifically, as shown in  FIG. 68  raw data at applied voltage of 0V is monitored for 4 minutes, the period of the noise at low-frequency range is accurately measured, and the frequency analysis based on Fourier transform is performed. 
     As illustrated in the Fourier-transformed data of  FIG. 68 , low frequency noises are present in a superimposed manner in a range below 0.5 Hz. In  FIG. 69 , the raw data of  FIG. 68  is noise-evaluated according to the JIS standard (a noise between 1 to 60 Hz exceeding 3 μVp-p does not occur more than once per second). It was found that the performance of 6 μVp-p, which is about twice the objective of 3 μVp-p, is obtained. As illustrated in  FIG. 70 , it is possible to apply a low-frequency cut-off to the obtained digital signal such that the noise would fall within 3 μVp-p so that the JIS standard for noise is satisfied. 
     The communication unit  16 R 2  is a communication interface, such as a communication chip, and has a function of communicating with another device by wireless communication or wired communication. For example, the communication unit  16 R 2  sends a potential signal and biological information to an external device, such as the terminal apparatus  14 , and receives information, such as control information, from an external device, such as the terminal apparatus  14 . 
     The communication may take place by connecting to plural devices simultaneously. As an example, an external sound input-output device, a data server, or devices of the same type (e.g. plural earphones) may be mutually connected with each other. If information regarding position or communication environment of the devices is available, it is possible to automatically assign a priority to those devices that are closed to each other, the devices that are within certain distances from each other or the devices having communication strength exceeding a certain level, when connecting these devices. If the state of the devices change during their operation, the devices may be deactivated in accordance with the change. Similarly, connection target devices may be changed automatically during their operation. 
     The first right-side electrode  16 R 3  is the electrode provided on the outer surface of the right-side earpiece  22 R. The right-side earpiece  22 R itself may form the first right-side electrode  16 R 3 . The second right-side electrode  16 R 4  is the electrode provided on the side surface of the right-side support  24 R, that is, the electrode formed by the electrode member  28 R. A potential signal indicating potentials detected by the first right-side electrode  16 R 3  and that by the second right-side electrode  16 R 4  are output to the processing unit  16 R 5 . 
     The first left-side electrode  16 L 3  is the electrode provided on the outer surface of the left-side earpiece  22 L. The left-side earpiece  22 L itself may form the first left-side electrode  16 L 3 . The second left-side electrode  16 L 4  is the electrode provided on the side surface of the left-side support  24 L, that is, the electrode formed by the electrode member  28 L. A potential signal indicating potentials detected by the first left-side electrode  16 L 3  and that by the second left-side electrode  16 L 4  are output from the left-side earphone  16 L to the processing unit  16 R 5  of the right-side earphone  16 R via the cable  18 . 
     The above-described number of electrodes is only an example. Multiple electrodes may be disposed in each of the right-side earpiece  22 R, the left-side earpiece  22 L, the electrode member  28 R, and the electrode member  28 L. 
     The processing unit  16 R 5  analyzes potential signals indicating potentials individually detected by the first right-side electrode  16 R 3 , the second right-side electrode  16 R 4 , the first left-side electrode  16 L 3 , and the second left-side electrode  16 L 4 . As a result of analyzing a potential signal, for example, the processing unit  16 R 5  extracts from the potential signal a specific bioelectric potential, such as a bioelectric potential having a specific frequency, eliminates noise, and then extracts brain waves, which are an example of biological information. The functions of the processing unit  16 R 5  are implemented by a processor, for example. A memory may also be used to implement the functions of the processing unit  16 R 5 . For example, the brain wave sensor  3410  may be used to implement the functions of the processing unit  16 R 5 . 
     The processing unit  16 R 5  may analyze potential signals indicating potentials detected by two or more electrodes selected from an electrode group including the first right-side electrode  16 R 3 , the second right-side electrode  16 R 4 , the first left-side electrode  16 L 3 , and the second left-side electrode  16 L 4 . 
     For example, the first right-side electrode  16 R 3  is used as a sensor electrode, the second right-side electrode  16 R 4  is used as a ground electrode, and the first left-side electrode  16 L 3  is used as a reference electrode. In this case, the processing unit  16 R 5  defines the potential detected by the second right-side electrode  16 R 4  to be a ground potential, which is a base potential, and calculates the potential difference between the potential detected by the first right-side electrode  16 R 3 , which is the sensor electrode, and the reference potential detected by the first left-side electrode  16 L 3 , which is the reference electrode, as a bioelectric potential. The processing unit  16 R 5  may execute known statistical processing on the potential difference and set the processing result to be biological information. A potential signal indicating a bioelectric potential and biological information are temporarily stored in the storage  16 R 6 , for example, and are then sent from the biological information monitoring apparatus  12  to an external device, such as the terminal apparatus  14 , by the communication unit  16 R 2 . Potential signals indicating potentials which have not been subjected to calculation or a potential signal indicating the potential difference which has not been subjected to statistical processing may be sent from the biological information monitoring apparatus  12  to an external device, such as the terminal apparatus  14 , and a potential difference may be calculated or statistical processing may be executed in the external device. 
     The storage  16 R 6  is constituted by a memory, for example. Potential signals indicating potentials detected by the first right-side electrode  16 R 3 , the second right-side electrode  16 R 4 , the first left-side electrode  16 L 3 , and the second left-side electrode  16 L 4  and information generated by the processing unit  16 R 5  may be stored in the storage  16 R 6 . 
     The battery  16 L 2  supplies power to the individual elements of the right-side earphone  16 R and the left-side earphone  16 L so as to drive them. As the battery  16 L 2 , a rechargeable/dischargeable battery is used. A non-rechargeable battery may alternatively be used. An electromagnetic-wave shielding member may be disposed around the battery  16 L 2  and charging-related parts. This can reduce noise caused by electromagnetic waves generated during charging. 
     The controller  16 R 7  controls the operations of the individual elements of the right-side earphone  16 R and the left-side earphone  16 L. The functions of the controller  16 R 7  are implemented by a processor, for example. A memory may also be used to implement the functions of the controller  16 R 7 . 
     The communication unit  16 R 2 , the processing unit  16 R 5 , the storage  16 R 6 , and the controller  16 R 7  may also be provided in the left-side earphone  16 L, while the battery  16 L 2  may also be provided in the right-side earphone  16 R. These components may be provided in either one of the right-side earphone  16 R and the left-side earphone  16 L. 
     The right-side earphone  16 R and the left-side earphone  16 L are connected with each other via the cable  18 . However, the cable  18  may not be included in the biological information monitoring apparatus  12 . In this case, a communication unit is also provided in the left-side earphone  16 L, and the right-side earphone  16 R and the left-side earphone  16 L communicate with each other (wirelessly) by using the individual communication units so as to send and receive data with each other. 
     Functions of the terminal apparatus  14  will be described below in detail with reference to the block diagram of  FIG. 25 . 
     A communication unit  14   a  is a communication interface, such as a communication chip, and has a function of communicating with another device by wireless communication or wired communication. 
     A storage  14   b  is a storage device, such as a hard disk drive or a memory. Various items of data and programs, for example, are stored in the storage  14   b.    
     A user interface (UI)  14   c  includes a display and an operation unit. The display is a liquid crystal display or an electroluminescence (EL) display, for example. The operation unit is an input unit, such as buttons, a keyboard, or a mouse. The UI  14   c  may be a touchscreen which serves both as the display and the operation unit. The UI  14   c  may include a microphone and/or a speaker. 
     A controller  14   d  controls the operations of the individual elements of the terminal apparatus  14 . The functions of the controller  14   d  are implemented by a processor, for example. A memory may also be used to implement the functions of the controller  14   d.    
     The processing unit  16 R 5  may be provided in the terminal apparatus  14 , and the terminal apparatus  14  may execute the above-described processing executed by the processing unit  16 R 5 . 
     The state of contact between the biological information monitoring apparatus  12 , which is a bearable device, and an ear of a user when the biological information monitoring apparatus  12  is worn on the ear will be described below in detail with reference to  FIGS. 26 through 28 .  FIGS. 26 and 28  schematically illustrate the external appearance of the right ear of a human.  FIG. 27  schematically illustrates the external appearance and the inside of the right ear of a human. 
     As shown in  FIG. 26 , an ear  60  includes an external acoustic opening  62  which leads to an ear canal  88  and parts around the external acoustic opening  62 , such as a cavity of concha  64 , a helix  66 , a concha of auricle  68 , a triangular fossa  70 , a scapha  72 , a pair of crura antihelices  74 , an antihelix  76 , a crus of helix  78 , a tragus  80 , an antitragus  82 , an intertragic notch  84 , and a lobule  86 . 
     The right-side earphone  16 R worn on the ear (right ear)  60  is shown in  FIG. 27 . A portion  90  of the electrode member  28 R brought into contact with the right ear  60  is shown in  FIG. 28 . 
     The right-side ear hanger  26 R is hung on the right ear  60 , and in this state, the right-side earpiece  22 R is inserted into the ear canal  88 . The outer surface of the right-side earpiece  22 R is then brought into contact with the surface (skin) of the ear canal  88 . The right-side earpiece  22 R, which is made of an elastic member, is deformed to match the shape of the ear canal  88 , which allows the outer surface of the right-side earpiece  22 R to closely contact the surface of the ear canal  88 . This achieves close contact between the electrode provided on the outer surface of the right-side earpiece  22 R and the surface of the ear canal  88 . 
     The electrode member  28 R contacts the cavity of concha  64 . The electrode member  28 R, which is made of an elastic member, is deformed to match the shape of the cavity of concha  64  and is brought into close contact with it. This achieves close contact between the electrode provided on the outer surface of the electrode member  28 R and the cavity of concha  64 , for example, the edge of the cavity of concha  64 . 
     When the right-side earphone  16 R configured as described above is worn on the right ear  60 , the movement thereof is restricted. This will be discussed below in detail. 
     The right ear  60  is positioned between the lower portion of the right-side ear hanger  26 R and the upper portion of the electrode member  28 R and is sandwiched therebetween in the top-bottom direction. This restricts the movement of the right-side earphone  16 R in the top-bottom direction. 
     The right portion of the right-side ear hanger  26 R contacts the helix of the right ear  60 , and a friction force is produced between the outer surface of the right-side earpiece  22 R and the ear canal  88 . This restricts the movement of the right-side earphone  16 R in the left-right direction. 
     The right ear  60  is sandwiched between the portion of the right-side ear hanger  26 R contacting the back side of the right ear  60  and the back portion of the electrode member  28 R in the front-back direction. This restricts the movement of the right-side earphone  16 R in the front-back direction. 
     In this manner, the movement of the right-side earphone  16 R is constrained. This can substantially maintain and stabilize the contact between the electrodes provided in the right-side earphone  16 R and the skin of the right ear  60 . It is thus possible to prevent or reduce the occurrence of noise mixed into bioelectric potentials, which would be caused by an unstable contact between the electrodes and the skin, thereby enhancing the accuracy in monitoring biological information. For example, even if a user having the biological information monitoring apparatus  12  mounted thereon moves, the movement of the right-side earphone  16 R is constrained as described above, thereby making it possible to prevent or reduce the occurrence of noise. 
     Depending on the shape of the ear or the head of a user, the right ear  60  (the base of the right ear  60 , for example) may be sandwiched between the lower portion of the right-side ear hanger  26 R and the upper portion of the right-side earpiece  22 R. This may also restrict the movement of the right-side earphone  16 R in the top-bottom direction. The right ear  60  (the base of the right ear  60 , for example) may also be sandwiched between the portion of the right-side ear hanger  26 R contacting the back side of the right ear  60  and the back portion of the right-side earpiece  22 R. This may also restrict the movement of the right-side earphone  16 R in the front-back direction. 
     As described above, the right-side earpiece  22 R, the electrode member  28 R, and the right-side ear hanger  26 R serve the function of positioning the right-side earphone  16 R. That is, these elements contribute to positioning the electrodes to be brought into contact with the skin by restricting the movements of the right-side earphone  16 R. 
     The left-side earphone  16 L is configured similarly to the right-side earphone  16 R. In the left-side earphone  16 L, the left-side earpiece  22 L is inserted into the ear canal of the left ear, and the electrode provided on the left-side earpiece  22 L is brought into contact with the surface of the ear canal. 
     The shapes of the ears are different depending on individual users. However, such individual differences can be absorbed by selecting a suitable size and shape of the right-side earpiece  22 R, the electrode member  28 R, the left-side earpiece  22 L, and/or the electrode member  28 L among different sizes and shapes. 
     The right-side earpiece  22 R may be swingable about the portion supported by the conductive tube member  30 R as the axis. This will be explained below with reference to  FIGS. 29 through 31 .  FIGS. 29 through 31  schematically illustrate the right-side earpiece  22 R and the conductive tube member  30 R. 
     As shown in  FIG. 29 , the right-side earpiece  22 R is provided at and supported by the forward end of the conductive tube member  30 R such that it is swingable in the direction indicated by the arrow X 1  in  FIG. 29 . For example, the right-side earpiece  22 R, which is made of conductive rubber, swings by utilizing its low stiffness (high flexibility). Alternatively, a spring member may be disposed in a coil-like shape or along the conductive tube member  30 R, thereby causing the right-side earpiece  22 R to swing. The left-side earpiece  22 L may also be swingable about the portion supported by a left-side conductive tube member as the axis. 
     In another example, as shown in  FIG. 30 , the conductive tube member  30 R itself may be swingable about the portion supported by the right-side housing  20 R, for example, as the axis in the direction indicated by the arrow X 1  in  FIG. 30 . Swinging the conductive tube member  30 R itself can tilt the right-side earpiece  22 R. For example, the conductive tube member  30 R may be made of a resin, such as rubber, or a spring member may be disposed in a coil-like shape or along the conductive tube member  30 R, thereby causing the conductive tube member  30 R to swing. The conductive tube member provided in the left-side earphone  16 L may also be swingable, as in the conductive tube member  30 R. 
     Tilting the right-side earpiece  22 R can adjust the angle of the surface of the right-side earpiece  22 R which contacts the ear canal. This makes it possible to bring the right-side earpiece  22 R into close contact with the ear canal, thereby enhancing the detection sensitivity of the electrode provided in the right-side earpiece  22 R. The shapes of the ears are different depending on individual users. However, such individual differences can be absorbed since the right-side earpiece  22 R tilts when being inserted into the ear canal. This achieves close contact between the electrode provided on the right-side earpiece  22 R and the ear canal. The left-side earpiece  22 L may be configured similarly to the right-side earpiece  22 R. 
     In another example, the member which contacts the cavity of concha may be movable. For example, as shown in  FIG. 31 , the electrode member  28 R which contacts the cavity of concha may be movable outwardly (in the direction separate from the ear canal, for example) and inwardly (in the direction closer to the ear canal, for example), as indicated by the arrow X 2  in  FIG. 31 . More specifically, the right-side support  24 R supporting the electrode member  28 R may be movable in the direction indicated by the arrow X 2  in  FIG. 31 . The right-side support  24 R may be swingable about the portion supported by the right-side housing  20 R as the axis in the direction indicated by the arrow X 3  in  FIG. 31 . Moving the electrode member  28 R makes it possible to bring the electrode member  28 R into close contact with the cavity of concha, thereby enhancing the detection sensitivity. The electrode member  28 L may also be configured similarly to the electrode member  28 R. More specifically, the left-side support  24 L may also be movable in the direction indicated by the arrow X 2  and be swingable in the direction indicated by the arrow X 3  in  FIG. 31 . 
     Another example of the earphone will be described below with reference to  FIG. 32 .  FIG. 32  is a sectional view of a right-side earphone  16 RA, which is an alternative to the right-side earphone  16 R. 
     As in the right-side earphone  16 R, the right-side earphone  16 RA includes the right-side housing  20 R, the right-side earpiece  22 R, and the right-side support  24 R. The right-side ear hanger  26 R may be provided or may not be provided in the right-side earphone  16 RA. 
     An electrode film  92 , which is a conductive film, is formed on the outer surface (that is, the surface facing the right ear) of the right-side support  24 R. The electrode film  92  is formed by coating the outer surface of the right-side support  24 R with a conductive paste. Instead of forming the electrode film  92 , the right-side support  24 R may be made of conductive rubber. 
     As in the right-side earphone  16 R, the right-side earpiece  22 R is made of conductive rubber. Alternatively, a conductive film may be formed on the outer surface of the right-side earpiece  22 R. 
     The right-side support  24 R is a hollow member and is provided on the right-side housing  20 R. A speaker  94  is disposed in the space of the right-side support  24 R and is fixed on the right-side housing  20 R. 
     One end of an electrical wire  96  is connected to the electrode provided in the right-side earpiece  22 R, and the other end thereof is connected to the substrate stored in the right-side housing  20 R. The electrical wire  96  is laid in the right-side support  24 R. If the right-side earpiece  22 R itself is made of conductive rubber, one end of the electrical wire  96  is connected to part of the right-side earpiece  22 R. If an electrode made of a conductive film, for example, is formed on the right-side earpiece  22 R, one end of the electrical wire  96  is connected to this electrode. 
     The left-side earphone is configured similarly to the right-side earphone  16 RA. 
     The electrode of the right-side earpiece  22 R is used as a sensor electrode or a reference electrode, and the electrode film  92  is used as a ground electrode, for example. If the electrode of the right-side earpiece  22 R is used as a sensor electrode, the electrode of the left-side earpiece  22 L is used as a reference electrode. If the electrode of the right-side earpiece  22 R is used as a reference electrode, the electrode of the left-side earpiece  22 L is used as a sensor electrode. 
     The speaker  94  is disposed separately from each of the electrodes (the electrode of the right-side earpiece  22 R and the electrode film  92 ). It is thus unlikely that noise caused by the speaker  94  will be mixed with potential signals indicating potentials detected by the electrodes. The use of fewer electrical wires also contributes to achieving high reliability. 
     The right-side earphone and the left-side earphone may be formed in a flexible structure. For example, the right-side earphone and the left-side earphone may be deformable to be adjusted to the shapes of the ears of a user. This will be discussed in detail with reference to  FIG. 33 . 
       FIG. 33  illustrates the external appearance of a right-side earphone  16 RB having a flexible structure. 
     The right-side earphone  16 RB includes a right-side earpiece  22 RB, an upper layer member  24 RB 1 , and a lower layer member  24 RB 2 . The upper layer member  24 RB 1  is connected to the lower layer member  24 RB 2 , and the right-side earpiece  22 RB is connected to the upper layer member  24 RB 1 . That is, the upper layer member  24 RB 1  is interposed between the lower layer member  24 RB 2  and the right-side earpiece  22 RB. 
     The upper layer member  24 RB 1  is a hollow member, and a conductive tube member for transmitting sound and electrical wires are disposed within the upper layer member  24 BR 1 . 
     The right-side earpiece  22 RB and the upper layer member  24 RB 1  are narrower than the lower layer member  24 RB 2 , for example. The right-side earpiece  22 RB is inserted into the ear canal and contacts the surface (that is, the skin) of the ear canal. The upper layer member  24 RB 1  contacts the cavity of concha. 
     The right-side earpiece  22 RB and the upper layer member  24 RB 1  are made of an elastic member, such as rubber or a sponge. The right-side earpiece  22 RB is flexible enough to be deformable to match the shape of the ear canal when being inserted into the ear canal. The upper layer member  24 RB 1  is flexible enough to be deformable when contacting the cavity of concha. 
     Each of the right-side earpiece  22 RB and the upper layer member  24 RB 1  may be made of conductive rubber. Alternatively, an electrode made of a conductive film may be formed on the surface of each of the right-side earpiece  22 RB and the upper layer member  24 RB 1 . Electrical wires connected to the individual electrodes are disposed within the upper layer member  24 RB 1 . 
     The lower layer member  24 RB 2  may be fixed on the right-side housing  20 RB. Elements such as substrates are housed in the right-side housing  20 RB. 
     The right-side earpiece  22 RB is flexible enough to be deformable to match the shape of the ear canal. This structure makes it possible to easily bring the electrode of the right-side earpiece  22 RB into close contact with the surface of the ear canal. The upper layer member  24 RB 1  is flexible enough to be deformable to match the shape of the cavity of concha. This structure makes it possible to easily bring the electrode of the upper layer member  24 RB 1  into close contact with the surface of the cavity of concha. 
     Modified examples of the earphone will be described below. 
     First Modified Example of Earphone 
     A first modified example of the earphone will be described below with reference to  FIGS. 34 and 35 .  FIG. 34  illustrates the external appearance of a left-side earphone  100 L according to the first modified example.  FIG. 35  illustrates the external appearance of a left ear  110 . 
     As shown in  FIG. 34 , the left-side earphone  100 L includes a left-side ear hanger  102 L and a left-side inserting member  104 L. 
     The left-side ear hanger  102 L has a curved portion so as to sandwich a helix  112  of the left ear  110 . An electrode  106 L is disposed on the inner side of the curved portion. For example, the left-side ear hanger  102 L is attached to the portion of the helix  112  which faces the back side of the head and sandwiches this portion. As a result of the curved portion touching the helix  112 , the electrode  106 L contacts the surface (that is, the skin) of the helix  112 . Instead of providing the electrode  106 L, the left-side ear hanger  102 L itself may be made of conductive rubber. 
     The left-side inserting member  104 L is formed in a cylindrical or semi-cylindrical shape and is connected to part of a first end  102 L 1  of the left-side ear hanger  102 L. The left-side inserting member  104 L is provided to project from the first end  102 L 1  of the left-side ear hanger  102 L so as to extend toward the left ear  110  when the left-side earphone  100 L is worn on the left ear  110 , that is, when the left-side ear hanger  102 L is attached to the helix  112 . 
     An electrode  108 L is provided on the side surface of the left-side inserting member  104 L. As shown in  FIG. 35 , the left-side inserting member  104 L is inserted into an ear canal  114 , and then, the electrode  108 L contacts the surface of the ear canal  114 . In the example in  FIG. 34 , the electrode  108 L is provided on the side surface of the left-side inserting member  104 L which is opposite the surface facing the left-side ear hanger  102 L. With this configuration, when the left-side earphone  100 L is worn on the left ear  110 , the electrode  108 L contacts the front portion of the ear canal  114 . The electrode  108 L may be provided at another portion of the side surface of the left-side inserting member  104 L or be provided on the entire side surface. Instead of providing the electrode  108 L, the left-side inserting member  104 L itself may be made of conductive rubber. 
     As indicated by the arrow X 4  in  FIG. 34 , sound emitted from a speaker disposed in the left-side earphone  100 L is output from the portion of the first end  102 L 1  which is not connected to the left-side inserting member  104 L. 
     When the left-side ear hanger  102 L is attached to the helix  112 , a second end  102 L 2  of the left-side ear hanger  102 L is positioned between the left ear  110  and the side face of the head (that is, at the base of the left ear  110 ). 
     The left-side ear hanger  102 L and the left-side inserting member  104 L may be formed in a flexible structure. For example, the left-side ear hanger  102 L may be flexible enough to be deformable to match the shape of the helix  112 . The left-side inserting member  104 L may be flexible enough to be deformable to match the shape of the ear canal  114 . 
     The right-side earphone is configured similarly to the left-side earphone  100 L. That is, the right-side earphone includes a right-side ear hanger with an electrode and a right-side inserting member with an electrode. 
     The electrode  106 L provided on the left-side hanger  102 L is used as a ground electrode, while the electrode  108 L provided on the left-side inserting member  104 L is used as a sensor electrode or a reference electrode. Likewise, the electrode provided on the right-side hanger is used as a ground electrode, while the electrode provided on the right-side inserting member is used as a sensor electrode or a reference electrode. For example, the electrode  108 L is used as a sensor electrode, while the electrode provided on the right-side inserting member is used as a reference electrode, or vice versa. 
     Another example of the left-side earphone  100 L is shown in  FIGS. 36 and 37 .  FIG. 36  illustrates the external appearance of another example of the left-side earphone  100 L.  FIG. 37  illustrates the external appearance of the left ear  110 . 
     In the example in  FIGS. 36 and 37 , the electrode  108 L is formed on the side surface of the left-side inserting member  104 L which faces the left-side ear hanger  102 L. With this configuration, as shown in  FIG. 37 , when the left-side earphone  100 L is worn on the left ear  110 , the electrode  108 L contacts the back portion of the ear canal  114 . The right-side earphone is configured similarly to the left-side earphone  100 L shown in  FIG. 36 . 
     A cable  116  is shown in  FIG. 36 . The cable  116  connects the left-side earphone  100 L and the right-side earphone with each other. The cable  116  may not be used, in which case, the left-side earphone  100 L and the right-side earphone communicate with each other wirelessly. A holder  118  may be provided in the cable  116 . In the holder  118 , the above-described various substrates housed in the right-side housing  20 R and the battery and other elements in the left-side housing  20 L may be stored. The substrates and the battery may alternatively be stored in the left-side earphone  100 L or the right-side earphone. Various switches and buttons may be provided in the holder  118 . 
     Second Modified Example of Earphone 
     A second modified example of the earphone will be described below with reference to  FIGS. 38 through 40 .  FIG. 38  illustrates a left-side earphone  120 L according to the second modified example as viewed from the left side.  FIG. 39  illustrates the left-side earphone  120 L as viewed from above.  FIG. 40  illustrates the external appearance of the left ear  110 . The left-side earphone  120 L of the second modified example is a bone-conduction earphone which transmits sound via bone conduction. 
     The left-side earphone  120 L includes a left-side hanger  122 L, a left-side support  124 L, a left-side bone conducting portion  126 L, and a left-side inserting member  128 L. In  FIG. 39 , the left-side hanger  122 L is not shown for the sake of representation. 
     The left-side ear hanger  122 L is generally formed in a curved shape so as to be hung on the left ear  110 . The left-side ear hanger  122 L is placed between the helix of the left ear  110  and the side face of the head (that is, at the base of the left ear  110 ) and is sandwiched therebetween. An electrode  130 L is provided on the inner side of the curved portion. As a result of the curved portion being placed between the helix and the side face of the head, the electrode  130 L contacts the base of the left ear  110 . Instead of providing the electrode  130 L, the left-side ear hanger  122 L itself may be made of conductive rubber. 
     One end of the left-side hanger  122 L is fixed to the left-side support  124 L. The left-side hanger  122 L is curved backward from the portion fixed to the left-side support  124 L so as to match the shape of the base of the left ear  110 , for example. 
     The left-side bone conducting portion  126 L is provided on part of the surface of the left-side support  124 L which faces the left ear  110  when the left-side earphone  120 L is worn on the left ear  110 . The left-side inserting member  128 L is provided to project from the surface of the left-side support  124 L toward the left ear  110 . As a result of the left-side bone conducting portion  126 L vibrating, sound is transmitted to the inner ear via bones (the skull, for example). An electrode  132 L is disposed on the side surface of the left-side inserting member  128 L. When the left-side earphone  120 L is worn on the left ear  110 , the left-side bone conducting portion  126 L contacts part of the left ear  110 . The left-side inserting member  128 L is inserted into the ear canal  114 , as shown in  FIG. 40 , and then, the electrode  132 L contacts the surface of the ear canal  114 . In the example in  FIGS. 39 and 40 , the electrode  132 L is provided on the side surface of the left-side inserting member  128 L which faces the back side of a user. With this arrangement, when the left-side earphone  120 L is worn on the left ear  110 , the electrode  132 L touches the back portion of the ear canal  114 . The electrode  132 L may be provided at another portion of the side surface of the left-side inserting member  128 L or be provided on the entire side surface. Instead of providing the electrode  132 L, the left-side inserting member  128 L itself may be made of conductive rubber. 
     When the left-side ear hanger  122 L is worn on the base of the left ear  110  and the left-side inserting member  128 L is inserted into the ear canal  114 , the left-side inserting member  128 L and the left-side ear hanger  122 L can sandwich the base of the left ear  110 . In this manner, the left-side earphone  120 L can be prevented from being displaced from a correct position. 
     The right-side earphone is configured similarly to the left-side earphone  120 L. That is, the right-side earphone includes a right-side ear hanger with an electrode and a right-side inserting member with an electrode. 
     The electrode  130 L provided on the left-side hanger  122 L is used as a ground electrode, while the electrode  132 L provided on the left-side inserting member  128 L is used as a sensor electrode or a reference electrode. Likewise, the electrode provided on the right-side hanger is used as a ground electrode, while the electrode provided on the right-side inserting member is used as a sensor electrode or a reference electrode. For example, the electrode  132 L is used as a sensor electrode, while the electrode provided on the right-side inserting member is used as a reference electrode, or vice versa. 
     A cable  134  is shown in  FIG. 38 . The cable  134  connects the left-side earphone  120 L and the right-side earphone with each other. The cable  134  may not be used, in which case, the left-side earphone  120 L and the right-side earphone communicate with each other wirelessly. A holder  136  may be provided in the cable  134 . In the holder  136 , the above-described various substrates housed in the right-side housing  20 R and the battery and other elements in the left-side housing  20 L may be stored. The substrates and the battery may alternatively be stored in the left-side earphone  120 L or the right-side earphone. Various switches and buttons may be provided in the holder  136 . 
     Third Modified Example of Earphone 
     A third modified example of the earphone will be described below with reference to  FIG. 41 .  FIG. 41  illustrates a right-side earphone  140 R according to the third modified example as viewed from the right side. The right-side earphone  140 R of the third modified example is a cartilage-conduction earphone which transmits sound via cartilage conduction. 
     The right-side earphone  140 R includes a right-side hanger  142 R, a right-side support  144 R, and a cartilage conducting portion  146 R. 
     The right-side ear hanger  142 R is generally formed in a curved shape so as to be hung on a right ear  148 . More specifically, the right-side ear hanger  142 R is hung on the top portion of a helix  150  of the right ear  148 . An electrode is provided on the inner side of the curved portion. When the curved portion is hung on the helix  150 , the electrode provided on the curved portion contacts the helix  150 . Instead of providing the electrode, the right-side ear hanger  142 R itself may be made of conductive rubber. 
     One end of the right-side hanger  142 R is fixed to the right-side support  144 R. The right-side support  144 R is disposed to contact the cavity of concha when the right-side earphone  140 R is worn on the right ear  148 . The cartilage conducting portion  146 R is provided at a portion of the right-side support  144 R which contacts the cavity of concha. As a result of the cartilage conducting portion  146 R vibrating, the cartilage in the ear canal also vibrates so as to transmit sound to the inner ear. 
     An electrode may be provided on the surface of the right-side support  144 R which contacts the surface of the cavity of concha of the right ear  148 . When the right-side earphone  140 R is worn on the right ear  148 , the right-side support  144 R contacts the cavity of concha, and more specifically, the electrode provided on the right-side support  144 R contacts the cavity of concha. 
     In another example, a right-side inserting member which is inserted into the canal of the right ear  148  may be provided at the surface of the right-side support  144 R. An electrode may be disposed on the side surface of the right-side inserting member. When the right-side earphone  140 R is worn on the right ear  148  and the right-side inserting member is inserted into the ear canal, the electrode provided on the right-side inserting member contacts the ear canal. The right-side inserting member may be an earpiece or a structure having a semi-cylindrical shape. 
     The left-side earphone is configured similarly to the right-side earphone  140 R. That is, the left-side earphone includes a left-side ear hanger with an electrode and a left-side support with an electrode. A left-side inserting member with an electrode may be provided on the left-side support. In this case, the provision of the electrode on the left-side support may be omitted. 
     The electrode provided on the right-side hanger  142 R is used as a ground electrode, while the electrode provided on the right-side support  144 R or the right-side inserting member is used as a sensor electrode or a reference electrode. Likewise, the electrode provided on the left-side hanger is used as a ground electrode, while the electrode provided on the left-side support or the left-side inserting member is used as a sensor electrode or a reference electrode. For example, the electrode provided on the right-side support  144 R or the right-side inserting member is used as a sensor electrode, while the electrode provided on the left-side support or the left-side inserting member is used as a reference electrode, or vice versa. 
     A cable  154  is shown in  FIG. 41 . The cable  154  connects the right-side earphone  140 R and the left-side earphone with each other. The cable  154  may not be used, in which case, the right-side earphone  140 R and the left-side earphone communicate with each other wirelessly. A holder may be provided in the cable  154 . In the holder, the above-described various substrates housed in the right-side housing  20 R and the battery and other elements in the left-side housing  20 L may be stored. The substrates and the battery may alternatively be stored in the right-side earphone  140 R or the left-side earphone. Various switches and buttons may be provided in the holder. 
     Fourth Modified Example of Earphone 
     A fourth modified example of earphone will be discussed below with reference to  FIGS. 76-80 .  FIG. 76  illustrates an earpiece  540  with two independent mushroom-shaped electrodes (the first electrode  521  and the second electrode  522 ) disposed on the earpiece axis  540 A. 
     The earpiece  540  may be two-flange or three-flange earpiece (such as that shown in  FIG. 77 ), where the conductive portions of the electrodes may be disposed in a staggered-manner on different flanges to obtain potentials separately, or a single-flange earpiece whose circumference spanning 360 degrees is divided into conductive regions and insulating regions to obtain potentials at the respective regions. A switching mechanism may also be disposed to switch among different electrodes. 
       FIG. 78  illustrates an example placement of the first electrode  521  and the second electrode  522  on the earpiece axis  520 A. The left side shows a view from above the earpiece axis  520 A and the right side shows a view from below the earpiece axis  520 A (or alternatively, a view when the earpiece axis  520 A is rotated 180 degrees around its axis). Similarly,  FIG. 79  ( 1 ) illustrates a view along the earpiece axis. The first electrode  521  is placed within a range of 180 degrees at the top side of the earpiece axis  520 A, and is for obtaining brain waves. The second electrode  522  is placed within a range of 180 degrees at the bottom side of the earpiece axis  520 A, and is for obtaining myoelectric potential. 
     This is explained in relation to  FIG. 80 , which shows an anatomy of the ear in relation to the brain and the jaw. Note that due to the bone structure, the ear canal consists of the external auditory aperture formed on the side skull (a tunnel-like aperture) and the skin. Thus, by placing the electrodes in the ear canal by the earpiece  520 , the top electrode (the first electrode  521 ) is placed in proximity to the side skull ( 600 A) so as to capture firing of neurons within the lateral lobe inside the skull, and the bottom electrode (the second electrode  522 ) is placed in proximity to the jaw ( 600 B) so as to capture myoelectric potentials such as those coming from the jaw muscles. 
     In this way, by placing two independent electrodes  521  and  522  in the ear canal, the brain waves and the myoelectric potential that are less contaminated by artifacts can be obtained. 
     The number of electrodes is also not limited to two. More than two electrodes can be placed on the earpiece, such as that illustrated in  FIG. 79  ( 2 ). 
     Note that depending on how the user wears the earpiece, the electrodes may not be placed at target positions. In such a case, an ear-hanger may be structured to guide the placement of the electrodes at target positions. For example, the ear-hanger or the earpiece may include adjustable structures such as a gear, a latch, a ratchet etc. so that the user may select and adjust the target locations at which the electrodes are placed. In this way, the brain waves and the myoelectric potential may be obtained with higher accuracy and with more flexibility. 
     In the case of simultaneously measuring the biological potentials at plural electrodes, separate data for each of the electrodes may be recorded. For instance, the measurement from the first electrode  521  is recorded as the first electrode data, and the measurement from the second electrode  522  is recorded as the second electrode data. 
     The above examples employed an earphone type device that covers the ear canal. However, other examples are now illustrated.  FIG. 72  illustrates a biological information monitoring apparatus  12  (a right and a left biological information monitoring apparatus  12 R ( 12 L)) in the form of a wireless ear accessory  400 R ( 400 L). The wireless ear accessories  400 R and  400 L each has a configuration as illustrated in  FIG. 71 , where the same reference numerals as in  FIG. 24  are used for the same or corresponding elements. The wireless ear accessory  400 R ( 400 L) can be worn only on one ear or as a pair of wireless ear accessories  400 R and  400 L worn independently on both ears. The respective housings for the right and the left ears includes substrates  34  and/or  36  illustrated in  FIG. 15  or a battery  38  illustrated in  FIG. 10 . The electrodes in the form of at least a sensor electrode and a common electrode that serves the both functions of a reference electrode and a ground electrode, or, preferably, in the form of three independent electrodes of a sensor electrode, a reference electrode and a ground electrode may be employed. The right and the left biological information monitoring apparatus  12  may also share the function of the reference electrode or the ground electrode of at least one of the right or the left biological information monitoring apparatus  12 . In the case of employing a common electrode for the reference electrode and the ground electrode, this may be realized by connecting a resistance, say, to the common electrode via a switching circuit. In this way, the resistance may be connected to the common electrode to realize a ground electrode, and disconnected to realize a reference electrode. In other words, potential difference may be realized during measurement using the switching circuit such that the common electrode operates as a differential amplifier. For example, plural channels may be provided for these three electrodes considered as a single set so that monitoring quality of the biological information can be enhanced. In this fashion, the biological information monitoring apparatus  12  may be made capable of being used on both the right and the left ears by use of both the right and the left biological information monitoring apparatuses ( 12 R,  12 L) or on one ear by use of one of the right or the left biological information monitoring apparatuses ( 12 R,  12 L), and in a case of being used on both the right and the left ears, the information may be obtained through plural (at least two) independent channels from the right and the left ears, and in a case of being used on one ear, the information may be obtained through at least one channel. Moreover, the information obtained from the right and the left ears may be compared to time signals of the corresponding right and the left biological information monitoring apparatuses ( 12 R,  12 L) and aligned sequentially in time. Alternatively, the information obtained may be aligned sequentially in time according to a time signal of one of the right or the left biological information monitoring apparatus ( 12 R,  12 L). Alternatively, the information obtained from the right and the left ears may be compared to a time signal of a common device to which the right and the left biological information monitoring apparatuses ( 12 R,  12 L) are connected. Moreover, when either or both of the left or the right biological information monitoring apparatuses ( 12 R,  12 L) are connected to an external device, modes of connection to an external device may be changed depending on a case in which the information is monitored from both the right and the left biological information monitoring apparatuses ( 12 R,  12 L) to output a single resulting information or a case in which the information is independently monitored from at least one of the right or the left biological information monitoring apparatuses ( 12 R,  12 L). An example of such a case is when Bluetooth (registered trademark) is employed, where for the latter case, the right and the left biological information monitoring apparatuses ( 12 R,  12 L) may respectively be recognized as independent devices when connecting via Bluetooth. 
       FIG. 75  illustrates an example positioning and attachment method for a case in which three electrodes  441 ,  442 , and  443  are provided. In the example of  FIG. 75 , all electrodes are positioned outside the ear (or outside an auricle of the ear). In the ear-hanger  420 R ( 420 L), as illustrated in  FIG. 74 , a spring structure  460  is placed inside a U-shaped structure of the ear-hanger  420 R ( 420 L). The ground electrode is placed at the position of the first electrode  441 , the reference electrode is placed at the position of the second electrode  442  and the sensor electrode is placed at the position of the third electrode  443 . Alternatively, the sensor electrode (the third electrode  443 ) may be in the form that can be placed in an ear canal as shown in  FIG. 73  ( 1 ) and as have been discussed in the Fourth Modified Example of Earphone with respect to  FIGS. 76-80 . Each of the three electrodes  441 ,  442  and  443  may be implemented in the form of a microneedle sheet or a pluripotent stem (iPS) cell sheet. 
     Alternatively, the U-shaped structure of the ear-hanger  420 R ( 420 L) may be made of shape memory alloy (e.g. titanium-nickel alloy, iron-magnesium alloy, resin, polymer textile or the like having shape-memory feature) so that a spring structure  460 , which is used to ensure the respective placements and contact points of the first electrode  441  and the second electrode  442 , is not needed. In such a case, the U-shaped structure may be made insulating so that the first electrode  441  and the second electrode  442  are not electrically connected. Furthermore, the U-shaped structure may deform or open in the direction of the arrow at around the body temperature such that the first electrode  441  and the second electrode  442  make close contact with the skin so that the respective electrode contact points are ensured. As to the extent of deformation, though not limited, materials that result in a distortion of 0.5 percent to 7.0 percent, or more preferably (from the perspective of durability and stable attachment) from 1.5 percent to 5 percent can be employed (for distortion, see https://www.yoshimi-inc.co.jp/development.php). In this way, the electrodes can be attached stably even when the wearer of the device is moving, so that monitoring quality loss due to floating electrodes may be prevented. 
     Furthermore, by forming the ear-hanger  420 R ( 420 L) itself by shape memory alloy, the third electrode  443  may also be fixed in close contact with the skin. In the same manner as with the U-shaped structure, when the ear-hanger  420 R ( 420 L) reaches the body temperature, the third electrode  443  may deform in the direction of the arrow towards and making close contact with the skin so that the third electrode  443  is stably attached. Moreover, the shape memory alloy may be employed on the external surface that contacts the user&#39;s skin or may be employed in the internal configuration of the ear-hanger. In either case, the contact points between the electrodes and the skin are improved. 
     Due to employing the structures and materials discussed above, the three electrodes  441 ,  442  and  443  can be simultaneously attached to make close contact with the monitoring target, enabling monitoring biological information in a stable manner. 
     Modified examples of an electrode used as a sensor electrode, a reference electrode, or a ground electrode will be described below. 
     First Modified Example of Electrode 
     An electrode according to a first modified example will be described below in detail with reference to  FIGS. 42A through 45 . The electrode according to the first modified example is a conductive microneedle electrode. 
       FIGS. 42A and 43  are perspective views illustrating a microneedle sheet  160 .  FIG. 42B  is a side view schematically illustrating a microneedle sheet  160 A.  FIG. 42C  is a schematic view illustrating the microneedle sheet  160 A as viewed from above. The microneedle sheet  160  is a sheet formed in a rectangle, for example. On one surface of the microneedle sheet  160 , multiple microneedles  162  constituted by conductive members are provided. Microneedles are needle-like members having a diameter or a length of 1 mm or smaller. Members similar to microneedles but having a size larger than 1 mm will be called projections in the exemplary embodiment. The microneedle sheet  160 A shown in  FIGS. 42B and 42C  have projections  162 B. The forward end of each projection  162 B is not pointed but is curved. The microneedles  162  may be made of a metal. However, for safety reasons, such as a breakage of the microneedles  162 , and for health reasons, such as allergic reactions, a material having a high biocompatibility, such as a biodegradable biopolymer having conductivity, is suitably used for the microneedles  162 . As the material for the projections  162 B, a conductive, elastic material may suitably be used. 
     As shown in  FIG. 43 , the microneedle sheet  160  is rolled up in a cylindrical shape with the surface having the microneedles  162  facing outward. 
       FIGS. 44A and 45  are sectional views schematically illustrating an ear canal  164 . As shown in  FIG. 44A , the microneedle sheet  160  rolled up in a cylindrical shape is attached to the forward end of a support  166  and is inserted into and placed in the ear canal  164 . For example, the microneedle sheet  160  is rolled up to a size small enough to be inserted into the ear canal  164 . It is appropriate that the microneedle sheet  160  be highly elastic and be flexibly deformed. To enhance the flexible deformation and the adhesiveness with the ear canal  164 , as shown in  FIG. 44B , the microneedle sheet  160  may be bonded onto an elastic base layer  160 C made of rubber, for example.  FIG. 44B  is a sectional view of the microneedle sheet  160  and the elastic base layer  160 C. The microneedle sheet  160  has a high elasticity and is flexibly deformed to follow various shapes of the ear canals of users, thereby enabling the microneedles  162  serving as an electrode to reliably contact the skin of the ear canal while enhancing the adhesiveness with the ear canal. Even with a motion of a user or a slight change in the shape of the ear canal caused by such a motion, the electrode is reliably brought into contact with the skin of the ear canal, thereby obtaining a stable electrical contact. The microneedle sheet  160  may be formed to be attachable to and detachable from the elastic base layer  160 C and be replaceable as consumables, thereby achieving a cost reduction. To facilitate the replaceability of microneedle sheets, instead of bonding the microneedle sheet  160  to the elastic base layer  160 C by the attraction of molecules on the surfaces of the two members due to physical and/or chemical force, non-adhesiveness characteristics may be utilized to easily detach the microneedle sheet  160  from the elastic base layer  160 C to prevent or eliminate the adhesiveness therebetween. 
     After the microneedle sheet  160  is inserted into and placed in the ear canal  164 , it is separated from the support  166  by cutting off the microneedle sheet  160 , for example. Then, the microneedle sheet  160  is spread in the ear canal  164  so as to contact the surface (that is, the skin) of the ear canal  164 . As shown in  FIG. 45 , the multiple microneedles  162  disposed on the surface of the microneedle sheet  160  contact and stick into the surface of the ear canal  164 . In this manner, the conductive microneedles  162  are placed in contact with the ear canal  164 . Unlike the microneedles  162 , the projections  162 B shown in  FIGS. 42B and 42C  do not stick into the skin, but they are deformed to contact the skin, thereby raising the contact pressure of the projections  162 B onto the ear canal  164 . 
     As shown in  FIG. 45 , a connector  168  constituted by a conductive member protrudes from one side of the microneedle sheet  160  (the portion fixed to the support  166 , for example). An electrical wire is formed on the microneedle sheet  160  and connects the individual microneedles  162  with each other. The electrical wire is also connected to the connector  168 . Hence, a potential signal indicating potentials detected by the conductive microneedles  162  can be output to the outside of the microneedle sheet  160  via the electrical wire and the connector  168 . 
     For example, a device body  170  is worn on the ear, as shown in  FIG. 45 . A connector receiver  172  is provided in the device body  170  to receive the connector  168 . When the device body  170  is worn on the ear, the connector  168  is fit into the connector receiver  172 . Then, a potential signal indicating potentials detected by the microneedles  162  is output to the device body  170 . 
     The microneedles  162  are used as a sensor electrode, a reference electrode, or a ground electrode. 
     Second Modified Example of Electrode 
     A second modified example of the electrode will be described below with reference to  FIG. 46 . In the second modified example, an induced pluripotent stem (iPS) cell sheet is used instead of the microneedle sheet  160 .  FIG. 46  is a perspective view illustrating an example of an iPS cell sheet  174 . The iPS cell sheet  174  is formed in a rectangle, for example, and an electrode  176  is formed on the surface of the iPS cell sheet  174 . A connector  178  connected to the electrode  176  is provided on the iPS cell sheet  174 . As in the microneedle sheet  160 , the iPS cell sheet  174  is rolled up in a cylindrical shape and is inserted into and placed in the ear canal  164 . The electrode  176  is thus brought into contact with the surface (that is, the skin) of the ear canal  164 . A potential signal indicating potentials detected by the electrode  176  is output to the device body  170  via the connector  178 . Instead of an iPS cell sheet, an electronic skin (also called an e-skin or a bionic skin) sheet may be employed. An electronic skin sheet is bonded to an elastic base layer made of rubber, for example. An electronic skin is flexibly deformed and reliably contacts the skin of the ear canal, thereby obtaining a stable electrical contact. For the electronic skin, a high polymer material having softness and elasticity equivalent to cells may suitably be used. For example, an electronic skin can be formed by constructing or printing an electronic circuit on a high polymer film substrate made of polyester or polyethylene terephthalate. 
     [Materials and Types of Electrodes] 
     Materials and types of electrodes for monitoring biological information (such as a sensor electrode, a reference electrode, and a ground electrode) will be discussed below. 
     Examples of the materials for the electrodes are gold (Au), platinum (Pt), silver (Ag), tungsten (W), molybdenum (Mo), copper (Cu), stainless steel (SUS304), solder, iron (Fe), and silver-silver chloride (Ag/AgCl). The electrode to be used may be changed in accordance with the type of biological information to be monitored. 
     Examples of the types of electrodes are: disposable surface electrodes using a conductive gel; dry electrodes, such as silver-silver chloride electrodes (Ag/AgCl electrodes), stainless steel electrodes, conductive rubber electrodes, and conductive high-polymer electrodes; non-contact electrodes; silver dish electrodes; suction electrodes; and clip electrodes. Other types of electrodes may be used. 
     Disposable surface electrodes do not need a paste and are easy to handle. Disposable surface electrodes also have a low impedance and a low offset voltage, thereby making monitoring stable. 
     Silver-silver chloride electrodes have a low floating potential and a low polarization voltage and can thus be stably utilized. The impedance of silver-silver chloride electrodes is up to several hundreds of kiloohms. 
     Stainless steel electrodes have a higher offset voltage than silver-silver chloride electrodes. 
     The impedance of conductive rubber electrodes and that of conductive high-polymer electrodes are several hundreds of kiloohms to several megaohms. Non-contact electrodes are used for monitoring an electrocardiographic signal and a myoelectric signal, for example. 
     [Charging Methods in Biological Information Monitoring Apparatus  12 ] 
     Charging methods for a battery provided in the biological information monitoring apparatus  12  will be described below. 
       FIG. 47  illustrates a head  180  of a user and a cushion  182  as viewed from above. An example of the charging method will be discussed below with reference to  FIG. 47 . Chargers  184  and  186  are disposed in the cushion  182 . The chargers  184  and  186  are wireless chargers including elements such as a power transmission coil so as to charge a battery in a non-contact manner. That is, the chargers  184  and  186  charge a battery by utilizing non-contact power transmission. The chargers  184  and  186  are disposed separately from each other in the cushion  182 . For example, the charger  184  is disposed closer to one end of the cushion  182  than the center thereof, while the charger  186  is disposed closer to the other end of the cushion  182  than the center thereof. The charger  184  charges a battery located within the wireless coverage area of the charger  184 . The charger  186  charges a battery located within the wireless coverage area of the charger  186 . 
     The right-side earphone  16 R is worn on the right ear of the user, while the left-side earphone  16 L is worn on the left ear of the user. Then, the head  180  is placed on the cushion  182 . The charger  184  is disposed at a position at which it can wirelessly charge a battery provided in one of the right-side earphone  16 R and the left-side earphone  16 L when the head  180  is placed on the cushion  182 . Likewise, the charger  186  is disposed at a position at which it can wirelessly charge a battery provided in one of the right-side earphone  16 R and the left-side earphone  16 L when the head  180  is placed on the cushion  182 . 
     In the example in  FIG. 47 , the right-side earphone  16 R is located within the wireless coverage area of the charger  184 . The charger  184  can thus wirelessly charge a battery provided in the right-side earphone  16 R. 
     In the example in  FIG. 47 , the left-side earphone  16 L is located within the wireless coverage area of the charger  186 . The charger  186  can thus wirelessly charge a battery provided in the left-side earphone  16 L. 
     Batteries provided in the right-side earphone  16 R and the left-side earphone  16 L each include elements such as a power receiving coil and can wirelessly be charged. 
     The charger  184  may start charging a battery when this battery is disposed within the wireless coverage area of the charger  184 . Similarly, the charger  186  may start charging a battery when this battery is disposed within the wireless coverage area of the charger  186 . 
     The charging timing and period may be controlled. For example, a control device may be disposed in the right-side earphone  16 R or the left-side earphone  16 L and may control the charging timing and period. The control device is implemented by an electronic circuit disposed on the main substrate or the sub-substrate, for example. Alternatively, the control device may be disposed in the cushion  182  and may control the charging timing and period. 
     For example, the control device may cause the chargers  184  and  186  to charge batteries when a user is asleep. The control device judges whether the user is asleep based on biological information, such as brain waves, monitored by the right-side earphone  16 R and the left-side earphone  16 L. 
     The control device may cause the charger  184  or  186  to charge a battery while biological information is being monitored. For example, a monitoring chip, such as a brain wave sensor, for monitoring biological information is provided only in the right-side earphone  16 R, while a battery is disposed only in the left-side earphone  16 L. In this manner, the battery and the monitoring chip are separately disposed in the different earphones. This enables the monitoring chip to monitor biological information without being influenced by wireless charging performed by the charger  184  or  186 . 
     The control device may cause the chargers  184  and  186  to charge batteries while a potential signal and biological information are not being sent. For example, a detected potential signal and biological information are sent from the right-side earphone  16 R to the terminal apparatus  14 . The control device does not cause the chargers  184  and  186  to charge batteries during the transmission of a detected potential signal and biological information. This makes it possible to send a potential signal and biological information without being influenced by wireless charging. 
     In the example in  FIG. 47 , three or more chargers may be disposed in the cushion  182 . 
     Another example of the charging method is shown in  FIG. 48 .  FIG. 48  illustrates the head  180  of a user and the cushion  182  as viewed from above. One charger  188  is disposed in the cushion  182 . The charger  188  is disposed at or near the center of the cushion  182 . If a battery is disposed in each of the right-side earphone  16 R and the left-side earphone  16 L, the charger  188  charges these batteries. The charger  188  may be formed in a size large enough to wirelessly charge batteries disposed in the right-side earphone  16 R and the left-side earphone  16 L, which are separated from each other, or may have a function of outputting magnetic fields or electric fields having an output level to wirelessly charge these batteries. Multiple chargers may be disposed in the cushion  182 . 
       FIG. 49  illustrates still another example of the charging method.  FIG. 49  illustrates the head  180  of a user and the cushion  182  as viewed from above. In the example in  FIG. 49 , a power receiving device  190  including a power receiving coil, for example, is provided at or near the center of the cable  18 . One charger  192  is disposed in the cushion  182 . The charger  192  is disposed at or near the center of the cushion  182 , for example. The power receiving device  190  supplies power received from the charger  192  to batteries provided in the right-side earphone  16 R and the left-side earphone  16 L so as to charge the batteries. 
       FIG. 50  illustrates still another example of the charging method.  FIG. 50  illustrates the head  180  of a user and a display  194  as viewed from above. A charger  196  is provided on one of the left and right edges (right edge, for example) of the screen of the display  194 , while another charger  198  is provided on the other one of the edges (left edge, for example) of the screen of the display  194 . 
     The charger  196  charges a battery located within the wireless coverage area of the charger  196 . The charger  198  charges a battery located within the wireless coverage area of the charger  198 . 
     In the state in which the user is looking at the screen of the display  194 , the right-side earphone  16 R worn on the right ear is located within the wireless coverage area of the charger  196 , while the left-side earphone  16 L worn on the left ear is located within the wireless coverage area of the charger  198 . The charger  196  can thus wirelessly charge a battery provided in the right-side earphone  16 R, while the charger  198  can wirelessly charge a battery provided in the left-side earphone  16 L. 
     The charger  196  may be disposed one edge (right edge, for example) of the top surface of the display  194 , while the charger  198  may be disposed on the other edge (left edge, for example) of the top surface of the display  194 . Three or more chargers may be provided on the display  194 . 
     In the example in  FIG. 50 , the above-described control device may be provided and control the charging timing. For example, the control device causes the chargers  196  and  198  to charge batteries when a user is still or is sitting in front of the screen  194  or when the motion of the user is smaller than or equal to a predetermined threshold, and stops the chargers  196  and  198  from charging batteries when the user is in a state other than the above-described states. For example, the control device stops the chargers  196  and  198  from charging batteries when the user is walking or doing some exercises. The motion of the user may be detected by a 6-axis sensor provided in the right-side earphone  16 R or by a camera disposed in the display  194 . The control device receives a signal indicating the motion of the user detected by the 6-axis sensor or that by the camera so as to detect the motion of the user. 
       FIG. 51  illustrates still another example of the charging method.  FIG. 51  illustrates the head  180  of a user and the display  194  as viewed from above. In the example in  FIG. 51 , one charger  200  is disposed at or near the center of the top surface of the display  194 . If a battery is disposed in each of the right-side earphone  16 R and the left-side earphone  16 L, the charger  200  charges these batteries. The charger  200  may be formed in a size large enough to wirelessly charge the batteries disposed in the right-side earphone  16 R and the left-side earphone  16 L, which are separately disposed from each other, or may have a function of outputting magnetic fields or electric fields having an output level to wirelessly charge these batteries. Multiple chargers may be provided on the display  194 . 
     In another example, a wireless charger may be provided on a chair. For example, a charger may be provided at the center or the left and right ends of the headrest of a chair. A charger may be provided at the center or the left and right ends of the top portion of the backrest of a chair. For example, the cushion  182  shown in  FIGS. 47 through 49  is replaced by a headrest or a backrest of a chair, and charging methods discussed with reference to  FIGS. 47 through 49  are employed. 
     In another example, a charger is provided in the terminal apparatus  14 , and the terminal apparatus  14  may charge a battery. 
     [Structure of Cable] 
     Various examples of a cable connecting the right-side earphone  16 R and the left-side earphone  16 L will be discussed below. 
     First Example of Cable 
       FIG. 52  is a sectional view illustrating the configuration of a cable  210  of a first example. The cable  210  includes electrical wires  212  through  222 . The electrical wire  212  is a wire for sending and receiving a potential signal indicating potentials detected by a sensor electrode or a reference electrode. Such an electrical wire will also be called a bioelectric potential wire. The electrical wire  214  is a wire for sending and receiving a potential signal indicating potentials detected by a ground electrode. Such a wire will also be called a ground electrode wire. The electrical wires  216  and  218  are wires for sending and receiving an audio signal indicating sound emitted from a speaker. Such a wire will also be called an audio signal wire. The electrical wires  220  and  222  are wires for sending and receiving power supplied from a battery. Such a wire will also be called a battery wire. To reduce noise, the electrical wires  212 ,  216 , and  218  are covered with shield lines. 
     For example, the electrode provided in the right-side earpiece  22 R of the right-side earphone  16 R is used as a sensor electrode, the electrode provided in the left-side earpiece  22 L of the left-side earphone  16 L is used as a reference electrode, and the electrode member  28 L of the left-side earphone  16 L is used as a ground electrode. The electrode member  28 R of the right-side earphone  16 R is not used. A monitoring chip, such as a brain wave sensor, for monitoring biological information is provided in the right-side housing  20 R of the right-side earphone  16 R. 
     In the above-described arrangement, a potential signal indicating potentials detected by the reference electrode provided in the left-side earphone  16 L is sent to the right-side earphone  16 R via the electrode wire  212  and is input into the monitoring chip, such as a brain wave sensor. A potential signal indicating potentials detected by the sensor electrode provided in the right-side earphone  16 R is input into the monitoring chip in the right-side earphone  16 R. 
     Second Example of Cable 
       FIG. 53  is a sectional view illustrating the configuration of a cable  230  of a second example. The cable  230  includes electrical wires  232 ,  234 ,  236 , and  238 . The electrical wire  232  is a wire for sending and receiving a potential signal indicating potentials detected by a sensor electrode or a reference electrode (bioelectric potential wire). The electrical wire  234  is a wire for sending and receiving a potential signal indicating potentials detected by a ground electrode (ground electrode wire). The electrical wire  236  is a wire for sending and receiving an audio signal (audio signal wire). The electrical wire  238  is a wire for sending and receiving power supplied from a battery (battery wire). 
     The cable  230  is formed in a circular or elliptical shape in cross section, for example. The electrical wires  232 ,  234 ,  236 , and  238  are disposed in the cable  230  in the peripheral direction. The bioelectric potential wire  232  is located farthest from the audio signal wire  236  so as to avoid the influence of noise caused by an audio signal. That is, the distance between the bioelectric potential wire  232  and the audio signal wire  236  is longer than that between the other wires. 
     Third Example of Cable 
       FIG. 54  is a sectional view illustrating the configuration of a cable  240  of a third example. The electrical wires  232 ,  234 ,  236 , and  238  discussed in the second example are disposed in the cable  240 . 
     The cable  240  is constituted by cable elements  242  and  244 , for example. The cable elements  242  and  244  are formed in a rectangular shape in cross section. 
     The electrical wires  236 ,  234 , and  238  are aligned in this order within the cable element  242 . The cable element  242  is longer than the cable element  244  in the longitudinal direction, that is, in the direction in which the electrical wires  236 ,  234 , and  238  are aligned. The cable element  244  projects from the cable element  242 . The electrical wire  232  is disposed in the cable element  244 . 
     The cable element  244  projects from the portion of the cable element  242  at which the ground electrode wire  234  is located. This makes the distance between the bioelectric potential wire  232  and the audio signal wire  236  longer than that in the configuration in which the audio signal wire  236  would be located at the position of the ground electrode wire  234 . It is thus possible to reduce the occurrence of noise in a bioelectric potential caused by an audio signal. Likewise, the distance between the bioelectric potential wire  232  and the battery wire  238  is longer than that in the configuration in which the battery wire  238  would be located at the position of the ground electrode wire  234 . It is thus possible to reduce the occurrence of noise in a bioelectric potential caused by power supply from a battery. 
     The cable  240  may be formed in a triangular shape in cross section. In this case, the electrical wires  232 ,  234 ,  236 , and  238  may be disposed with the arrangement shown in  FIG. 54  in the triangular cross section. 
     Fourth Example of Cable 
       FIG. 55  is a sectional view illustrating the configuration of a cable  250  of a fourth example. The electrical wires  232 ,  234 ,  236 , and  238  discussed in the second and third examples are disposed in the cable  250 . 
     The electrical wires  232 ,  234 ,  236 , and  238  are aligned in this order in the cable  250 . The order of the electrical wires  232 ,  234 ,  236 , and  238  is not restricted to that shown in  FIG. 55 . The bioelectric potential wire  232  may be disposed separately from the battery wire  238 , thereby reducing the occurrence of noise in a bioelectric potential caused by power supply. Likewise, the bioelectric potential wire  232  may be disposed separately from the audio signal wire  236 , thereby reducing the occurrence of noise in a bioelectric potential caused by an audio signal. 
     Fifth Example of Cable 
       FIG. 56  is a sectional view illustrating the configuration of a cable  260  of a fifth example. The electrical wires  232 ,  234 ,  236 , and  238  discussed in the second through fourth examples are disposed in the cable  260 . 
     The cable  260  is constituted by cable elements  262  and  264 , for example. The cable elements  262  and  264  are formed in a rectangular shape in cross section. 
     The electrical wires  232 ,  234 , and  236  are aligned in this order within the cable element  262 . The cable element  262  is longer than the cable element  264  in the longitudinal direction, that is, in the direction in which the electrical wires  232 ,  234 , and  236  are aligned. The cable element  264  projects from the cable element  262 . The electrical wire  238  is disposed in the cable element  264 . The cable element  264  projects from the portion of the cable element  262  at which the audio signal wire  236  is located. This makes it possible to separately dispose the bioelectric potential wire  232  from the audio signal wire  236  and thus to reduce the occurrence of noise in a bioelectric potential caused by an audio signal. 
     Sixth Example of Cable 
       FIG. 57  is a sectional view illustrating the configuration of a cable  270  of a sixth example. The electrical wires  232 ,  234 ,  236 , and  238  discussed in the second through fifth examples are disposed in the cable  270 . 
     A cable  270  is a coaxial cable and is formed in a circular, elliptical, or rectangular shape in cross section. The ground electrode wire  234  is disposed around the bioelectric potential wire  232 . The audio signal wire  236  is disposed around the ground electrode wire  234 . The battery wire  238  is disposed around the audio signal wire  236 . The ground electrode wire  234  is interposed between the bioelectric potential wire  232  and the audio signal wire  236  and also between the bioelectric potential wire  232  and the battery wire  238 . In other words, the bioelectric potential wire  232  is not located adjacent to the audio signal wire  236  or the battery wire  238 . It is thus possible to reduce the occurrence of noise in a bioelectric potential caused by an audio signal or power supply. 
     Seventh Example of Cable 
       FIG. 58  is a sectional view illustrating the configuration of a cable  280  of a seventh example. The electrical wires  232 ,  234 ,  236 , and  238  discussed in the second through sixth examples are disposed in the cable  280 . 
     A coaxial cable is constituted by the ground electrode wire  234 , the audio signal wire  236 , and the battery wire  238 . The bioelectric potential wire  232  is not included in the coaxial cable but is separately disposed at another position in the cable  280 . As a result of separately disposing the bioelectric potential wire  232  from the other electrical wires, the occurrence of noise in a bioelectric potential caused by an audio signal or power supply is reduced. 
     Eighth Example of Cable 
       FIG. 59  is a schematic view illustrating a cable of an eighth example. The right-side earphone  16 R, the left-side earphone  16 L, and the cable  18  are schematically shown in  FIG. 59 . 
     The right-side earphone  16 R and the left-side earphone  16 L may be attachable to and detachable from the cable  18 . As connector terminals, known standards, such as USB or micro-miniature coaxial (MMCX) connector terminals, may be used. For example, the right-side earphone  16 R and the left-side earphone  16 L may be separated from the cable  18  and be connected to another cable. 
     When the right-side earphone  16 R and the left-side earphone  16 L are connected to the cable  18 , they may communicate with another device, such as the terminal apparatus  14 , by wired communication. When the right-side earphone  16 R and the left-side earphone  16 L are disconnected from the cable  18 , they may communicate with another device by wireless communication. That is, the communication mode may automatically be switched from the wired communication mode to the wireless communication mode. The right-side earphone  16 R and the left-side earphone  16 L may be driven independently. 
     The functions of the right-side earphone  16 R and the left-side earphone  16 L and the communication mode may be changed in accordance with the type of cable  18  connected thereto. 
     Two devices of the same type may be connected to the cable  18 , or two devices of different types may be connected to the cable  18 . For example, the device connected to one end of the cable  18  may serve as a main device, while that to the other end of the cable  18  may serve as a sub-device. In this case, the main device may control the sub-device. 
     If two devices of different types are connected to the cable  18 , the functions of these devices may be restricted. For example, access to memory units provided in the devices may be restricted, or it may not be allowed to control the devices. 
     The electrodes provided in the device connected to one end of the cable  18  may be used as a reference electrode and a ground electrode, while the electrode provided in the device connected to the other end of the cable  18  may be used as a sensor electrode. For example, when the right-side earphone  16 R is connected to one end of the cable  18 , the electrode of the right-side earpiece  22 R is used as a reference electrode and the electrode member  28 R is used as a ground electrode. When the left-side earphone  16 L is connected to the other end of the cable  18 , the electrode of the left-side earpiece  22 L is used as a sensor electrode. For example, a brain wave sensor handles a potential signal indicating potentials detected by the electrode of the left-side earpiece  22 L as a potential signal from the sensor electrode, a potential signal indicating potentials detected by the electrode of the right-side earpiece  22 R as a potential signal from the reference electrode, and a potential signal indicating potentials detected by the electrode member  28 R as a potential signal from the ground electrode. 
     Processing to be executed for a potential signal indicating potentials detected by each electrode will be described below, assuming that brain waves are monitored as biological information. As an example, processing for generating a brain wave signal based on sensor potentials detected by a sensor electrode, reference potentials detected by a reference electrode, and ground potentials detected by a ground electrode will be described below. 
     A balanced differential amplifier is used for generating a brain wave signal. For example, the above-described brain wave sensor  3410  (see  FIG. 16 ) implements the function of a balanced differential amplifier. As the brain wave sensor  3410 , a known balanced differential amplifier may be used. 
       FIG. 60  illustrates the configuration of the balanced differential amplifier. A potential signal indicating sensor potentials is input into the negative terminal of amplifier  1  (input  1 ), while a potential signal indicating reference potentials is input into a negative terminal of amplifier  2  (input  2 ). A potential signal indicating ground potentials is input into the positive terminal of each of amplifier  1  and amplifier  2 . The potential signal is amplified in each of amplifier  1  and amplifier  2 , and the difference between a signal output from amplifier  1  (output  1 ) and a signal output from amplifier  2  (output  2 ) is obtained. This difference output represents a brain wave signal. 
     Components (brain waves, for example) different between the potential signal indicating the sensor potentials and the potential signal indicating the reference potentials are amplified, while components common to the two potential signals (AC noise, for example) are canceled out. 
     [Selection of Electrodes] 
     Selection of a sensor electrode, a reference electrode, and a ground electrode will be described below through illustration of examples. 
     For example, a sensor electrode and a reference electrode are placed in the ears to contact the ear canals, while a ground electrode is placed in the ear to contact the cavity of concha. It is assumed, for example, that potential signals indicating potentials detected by the electrodes are output to a brain wave sensor. 
     First Example of Selection of Electrodes 
     A first example of the selection of electrodes will be discussed below with reference to  FIG. 61 .  FIG. 61  is a diagram for explaining how to select electrodes. 
     The electrode placed in the ear canal of the right ear (electrode of the right-side earpiece  22 R, for example) is used as a sensor electrode. The electrode placed in the ear canal of the left ear (electrode of the left-side earpiece  22 L, for example) is used as a reference electrode. The electrode placed in the cavity of concha of the right ear (electrode member  28 R, for example) is used as a ground electrode. The electrode placed in the cavity of concha of the left ear (electrode member  28 L, for example) is used as a ground electrode. 
     A potential signal indicating potentials detected by the sensor electrode and that by the reference electrode are output to the brain wave sensor. 
     One of a potential signal indicating potentials detected by the ground electrode placed in the cavity of concha of the right ear and that by the ground electrode placed in the cavity of concha of the left ear is output to the brain wave sensor, and the other potential signal is not output. The controller  16 R 7  (see  FIG. 24 ), for example, selects which one of the potential signals is output to the brain wave sensor. For example, out of the two ground electrodes, the ground electrode that detects more stable potentials is selected, and a potential signal indicating potentials detected by the selected ground electrode is output to the brain wave sensor. A potential signal indicating more stable potentials is a signal having a smaller amplitude and a smaller change in the amplitude. For example, the right-side ground electrode and the left-side ground electrode are electrically connected with each other, and then, the ground electrode which detects potentials with a lower level of noise (potentials having a smaller amplitude) is selected. A potential signal indicating potentials detected by the selected ground electrode is output to the brain wave sensor. 
     Selection of electrodes may be performed before starting to monitor biological information. If a potential signal indicating potentials detected by a selected electrode is found to be unstable, the electrode to be used may be changed. This is also applied to the following examples. 
     Second Example of Selection of Electrodes 
     A second example of the selection of electrodes will be discussed below with reference to  FIG. 62 .  FIG. 62  is a diagram for explaining how to select electrodes. 
     Four electrodes (electrodes A, B, C, and D, for example) are placed in the ear canal of the right ear and are used as sensor electrodes. The electrode placed in the ear canal of the left ear is used as a reference electrode. The electrode placed in the cavity of concha of the left ear is used as a ground electrode. 
     A potential signal indicating potentials detected by the reference electrode and a potential signal indicating potentials detected by the ground electrode are output to the brain wave sensor. 
     A potential signal indicating potentials detected by one of the electrodes A, B, C, and D is output to the brain wave sensor. Among the electrodes A, B, C, and D, the electrode which detects more stable potentials is selected, and a potential signal indicating potentials detected by the selected electrode is output to the brain wave sensor. For example, a potential signal having a smaller amplitude of noise is selected as a signal indicating more stable potentials. The controller  16 R 7 , for example, selects which one of the potential signals is output to the brain wave sensor. 
     Third Example of Selection of Electrodes 
     A third example of the selection of electrodes will be discussed below with reference to  FIG. 63 .  FIG. 63  is a diagram for explaining how to select electrodes. 
     Two electrodes (electrodes A and B, for example) are placed in the ear canal of the right ear and are used as sensor electrodes. Two electrodes (electrodes C and D, for example) are placed in the ear canal of the left ear and are used as reference electrodes. The electrode placed in the cavity of concha of each of the left and right ears is used as a ground electrode. 
     Out of the electrodes A and B, the electrode which detects more stable potentials is selected, and a potential signal indicating potentials detected by the selected electrode is output to the brain wave sensor. For example, a potential signal having a smaller amplitude of noise is selected as a signal indicating more stable potentials. 
     Out of the electrodes C and D, the electrode which detects more stable potentials is selected, and a potential signal indicating potentials detected by the selected electrode is output to the brain wave sensor. For example, a potential signal having a smaller amplitude of noise is selected as a signal indicating more stable potentials. 
     Out of the left-side ground electrode and the right-side ground electrode, the electrode which detects more stable potentials is selected, and a potential signal indicating potentials detected by the selected ground electrode is output to the brain wave sensor. For example, a potential signal having a smaller amplitude and a smaller change in the amplitude is selected as a signal indicating more stable potentials. 
     [Multiple Channels] 
     Plural items of the same type of biological information may be monitored. One item of biological information corresponds to one channel and multiple channels are formed in accordance with plural items of biological information. Processing for multiple channels of biological information will be described below with reference to the block diagram of  FIG. 64 . 
     The functions shown in  FIG. 64  are implemented by the brain wave sensor  3410  (see  FIG. 16 ) or the processing unit  16 R 5 . 
     Another device or another processor may be used to implement the functions. It is assumed that the functions are implemented by the brain wave sensor  3410 . 
     For example, a sensor electrode, a reference electrode, and a ground electrode are placed in the right ear. Biological information indicating brain waves is monitored by these electrodes and is processed by the brain wave sensor  3410  as a signal of channel CH 1 . 
     Additionally, a sensor electrode, a reference electrode, and a ground electrode are placed in the left ear. Biological information indicating brain waves is monitored by these electrodes and is processed by the brain wave sensor  3410  as a signal of channel CH 2 . 
     The above-described items of biological information allocated to channels CH 1  and CH 2  are only examples. Biological information monitored by an electrode placed in another part may be allocated to channel CH 1  or channel CH 2 . 
     A selector  300  receives biological information of channel CH 1  and that of channel CH 2 . The selector  300  switches between channels CH 1  and CH 2  at predetermined intervals and outputs one of biological information of channel CH 1  and that of channel CH 2  to a signal storage  302 . 
     The signal storage  302  stores biological information output from the selector  300 . For example, biological information of channel CH 1  is stored in the signal storage  302  in association with channel information indicating channel CH 1 . Biological information of channel CH 2  is stored in the signal storage  302  in association with channel information indicating channel CH 2 . A signal  304 , which is an example of biological information, is stored in the signal storage  302 . 
     A signal separator  306  extracts biological information stored in the signal storage  302 , separates it into biological information of channel CH 1  and that of channel CH 2 , and separately outputs these items of biological information. 
     While channel CH 1  is being selected by the selector  300  and biological information of channel CH 1  is being output to the signal storage  302 , biological information of channel CH 2  is not output to the signal storage  302 . Likewise, while channel CH 2  is being selected by the selector  300  and biological information of channel CH 2  is being output to the signal storage  302 , biological information of channel CH 1  is not output to the signal storage  302 . In this manner, as a result of the selector  300  switching between the channels, while one channel is not being selected, biological information of this channel is not stored in the signal storage  302 . 
     For each channel, the signal separator  306  estimates biological information of a channel which is not being selected (that is, biological information of the channel which is not stored in the signal storage  302 ), based on biological information of this channel obtained during another period for which the channel is selected, thereby interpolating biological information during the period for which the channel is not selected. 
     For example, the signal separator  306  interpolates biological information of channel CH 1  which is not being selected, based on biological information of channel CH 1  obtained during the preceding and following periods for which channel CH 1  is selected. Likewise, the signal separator  306  interpolates biological information of channel CH 2  which is not being selected, based on biological information of channel CH 2  obtained during the preceding and following periods for which channel CH 2  is selected. 
     A signal  308  shown in  FIG. 64  is an example of biological information of channel CH 1 . The signal  308  includes signal components  308   a  and  308   b . The signal component  308   a  indicated by the solid line represents biological information of channel CH 1  which is selected by the selector  300 . The signal component  308   b  indicated by the broken line represents biological information of channel CH 1  which is not selected by the selector  300 , and this biological information is interpolated based on the signal component  308   a  during the preceding and following periods for which channel CH 1  is selected. 
     A signal  310  shown in  FIG. 64  is an example of biological information of channel CH 2 . The signal  310  includes signal components  310   a  and  310   b . The signal component  310   a  indicated by the solid line represents biological information of channel  2  which is selected by the selector  300 . The signal component  310   b  indicated by the broken line represents biological information of channel CH 2  which is not selected by the selector  300 , and this biological information is interpolated based on the signal component  310   a  during the preceding and following periods for which channel CH 2  is selected. 
     In addition to or instead of switching between the channels at predetermined intervals, the selector  300  may switch between the channels based on the factor other than the time. That is, the time may not necessarily be used for switching between the channels. 
     For example, the selector  300  may switch between the channels in accordance with the magnitude of noise of each channel. More specifically, the selector  300  may select a channel having a smaller amplitude of noise and output biological information of the selected channel to the signal storage  302 . In another example, when executing an operation or processing using a right brain signal, the selector  300  selects channel CH 1  and outputs biological information of channel CH 1  to the signal storage  302 . For example, in response to an instruction to operate a device by using a right brain signal, the selector  300  selects channel CH 1 . Likewise, when executing an operation or processing using a left brain signal, the selector  300  selects channel CH 2  and outputs biological information of channel CH 2  to the signal storage  302 . 
     In another example, the selector  300  may select a channel in accordance with an instruction from a user. 
     [Noise Cancelling] 
     Processing for eliminating noise from biological information, that is, noise cancelling processing, will be discussed below. Noise cancelling processing is executed by the processing unit  16 R 5 , for example. Noise refers to potential signal information other than that of a target subject to be monitored. 
     For example, from biological information monitored by electrodes and sensors for monitoring biological information of a target subject, the processing unit  16 R 5  eliminates biological information monitored by the other electrodes and sensors, thereby generating noise-free biological information of the target subject. That is, the processing unit  16 R 5  handles biological information monitored by the other electrodes and sensors as noise and eliminates the noise from biological information monitored by the electrodes and sensors for monitoring biological information of the target subject, thereby generating noise-free biological information of the target subject. 
     This will be explained below, assuming that brain waves are the target subject to be monitored. From biological information including information indicating brain waves (differential output shown in  FIG. 60 , for example) monitored by the above-described sensor electrode, reference electrode, and ground electrode, the processing unit  16 R 5  eliminates biological signals (also called biosignals) obtained from the other electrodes and sensors, such as a pulse sensor and an acceleration sensor, thereby generating noise-free biological information of the target subject. For example, from biological information including information indicating brain waves, the processing unit  16 R 5  eliminates the periods and the impulses of biological signals obtained from the other electrodes and sensors, such as a pulse sensor and an acceleration sensor, as noise. Noise refers to a signal transmitted through an organism and a signal transmitted from the outside of an organism, for example. 
     For example, the processing unit  16 R 5  handles as noise biological information monitored by electrodes and sensors placed in parts other than the parts on which a sensor electrode, a reference electrode, and a ground electrode are disposed. For example, when brain waves are monitored by using electrodes disposed in the ear canal, a myoelectric signal generated by pulsation (the blood flow cycle of blood vessels, for example), the motion of jaws, and blinking is treated as noise. The processing unit  16 R 5  eliminates this myoelectric signal from a biological signal obtained by the sensor electrode, reference electrode, and ground electrode, thereby generating a noise-free brain wave signal indicating brain waves. 
     This will be explained more specifically. In the ear canal, electrode A is placed at a top position (that is, a position close to the head), electrode B is placed at a bottom position (that is, a position close to the jaws), electrode C is placed at a front position (that is, a position close to the face), and electrode D is placed at a back position (that is, a position close to the body). The potential detected by the electrode C at a front position is likely to reflect myoelectric potentials caused by the motion of the face, such as blinking. The potential detected by the electrode B at a bottom position is likely to reflect myoelectric potentials caused by the motion of jaws and swallowing. In this case, the processing unit  16 R 5  handles the potentials detected by the electrodes B and C as noise and eliminates such potentials from biological information obtained from potentials detected by the other electrodes (differential output in  FIG. 60 , for example). As a result, a brain wave signal representing brain waves, free from noise caused by the motion of the face, jaws, and swallowing, can be obtained. 
     Noise cancelling processing has been discussed through illustration of brain waves. When another biological information is monitored, noise is eliminated similarly to that from brain waves. 
     Modified Examples of User Interface 
     The biological information monitoring apparatus  12  may display various items of information (such as biological information and other information) at a place other than the biological information monitoring apparatus  12 . For example, the biological information monitoring apparatus  12  may display various items of information on an organism or an object, such as a desk or a wall. 
     If the biological information monitoring apparatus  12  is a watch-type wearable device worn on the arm of a user, it may display various items of information on the arm or the back of the hand of the user. 
     If the biological information monitoring apparatus  12  is a pair-of-glasses-type wearable device, it may display various items of information on the glasses. 
     Example of Sharing of Biological Information Monitoring Apparatus  12   
     The biological information monitoring apparatus  12  may be shared among multiple users. For example, after user A has used the biological information monitoring apparatus  12 , user B may use it. 
     It is assumed that a contact-type biological information monitoring apparatus  12 , which monitors biological information by brining electrodes and sensors into contact with an organism, such as a human, is used. When the organism (user) using the biological information monitoring apparatus  12  is changed by another organism (user), for example, the biological information monitoring apparatus  12  may output information that the user has been changed or an instruction to clean the biological information monitoring apparatus  12 . Outputting such information or an instruction is performed by displaying such information or an instruction on a display, emitting it from a speaker as sound, or both operations. For example, if the right-side earphone  16 R and the left-side earphone  16 L are used, the controller  16 R 7  performs control to emit such information or an instruction from these earphones as sound. The controller  16 R 7  may alternatively perform control to display such information or an instruction on a display provided in the biological information monitoring apparatus  12 . Alternatively, the controller  14   d  of the terminal apparatus  14  may perform control to display such information or an instruction on the terminal apparatus  14  or to emit it from a speaker provided in the terminal apparatus  14  as sound. 
     If a microphone is provided in the biological information monitoring apparatus  12 , the controller  16 R 7  may recognize a user using the biological information monitoring apparatus  12  based on the voice of this user input from the microphone, and judge whether the user has changed. Alternatively, the controller  16 R 7  may recognize the user based on an image generated by a camera provided in the terminal apparatus  14 . 
     In another example, the accounts of users using the biological information monitoring apparatus  12  are managed by the biological information monitoring apparatus  12 , the terminal apparatus  14 , or a server. When the account of the user using the biological information monitoring apparatus  12  is switched to another account, the controller  16 R 7  judges that the user  12  has changed. 
     Outputting information that the user using the biological information monitoring apparatus  12  has changed allows a new user to recognize that the biological information monitoring apparatus  12  has been used by another user and needs cleaning. Outputting an instruction to clean the biological information monitoring apparatus  12  also gives this information to a new user. 
     A sensor for sensing that the biological information monitoring apparatus  12  is cleaned may be used. For example, a sensor for sensing that electrodes and sensors are cleaned with alcohol (a sensor for sensing alcohol, for example) is used. When a user is trying to use the biological information monitoring apparatus  12  with electrodes and sensors uncleaned (when the uncleaned biological information monitoring apparatus is powered ON, for example), the controller  16 R 7  outputs information that the biological information monitoring apparatus  12  has not been cleaned. The controller  16 R 7  may prevent the biological information monitoring apparatus  12  from being powered ON. When the user is trying to use the biological information monitoring apparatus  12  with the electrodes and sensors cleaned, the controller  16 R 7  may output information that the biological information monitoring apparatus  12  has been cleaned or may not output such information. 
     In the embodiments above, the term “processor” refers to hardware in a broad sense. Examples of the processor include general processors (e.g., CPU: Central Processing Unit) and dedicated processors (e.g., GPU: Graphics Processing Unit, ASIC: Application Specific Integrated Circuit, FPGA: Field Programmable Gate Array, and programmable logic device). In the embodiments above, the term “processor” is broad enough to encompass one processor or plural processors in collaboration which are located physically apart from each other but may work cooperatively. The order of operations of the processor is not limited to one described in the embodiments above, and may be changed. 
     The foregoing description of the exemplary embodiments of the present disclosure has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical applications, thereby enabling others skilled in the art to understand the disclosure for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the following claims and their equivalents.