Abstract:
The invention relates to a multipolar electrical connector, and more particularly to a multipolar electrical connector that, as a counter connector  90 , uses a single-head plug having a pole shaft  91  in which center poles are disposed in plural positions in the axial direction, respectively (hereinafter, referred to merely as “multipolar connector”). In the invention, plural predetermined contact pieces are incorporated into the body  10 , so that plural poles are ensured by the predetermined contact pieces. In the invention, a required number of first additional contact pieces  40  and second additional contact pieces  50  are placed on the outer and inner peripheral sides of the boss portion  30 , so that two or more poles are ensured by the additional contact pieces  40  and  50 . According to the invention, therefore, the number of poles is increased by that of poles which are formed by the additional contact pieces. Furthermore, the number of poles is increased by using the boss portion protruding from the body  10 . Even when the number of poles is increased, therefore, the size of the whole of the multipolar connector is not increased in proportion to the increased number of poles.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multipolar electrical connector, and more particularly to a multipolar electrical connector that, as a counter connector, uses a single-head plug having a pole shaft in which center poles are disposed in plural positions in the axial direction, respectively (hereinafter, such a multipolar electrical connector is referred to merely as “multipolar connector”). The invention relates also to a multipolar connector wherein a boss portion protrudes from the body to which a predetermined contact piece is attached, and contact pieces of two or more poles can be added by using the boss portion. A multipolar connector of this kind can be preferably used in the field of a mobile communication apparatus such as a portable telephone, and that of an electric or electronic apparatus to which a portable cassette tape recorder, a radio receiver, and the like belong. 
     2. Description of the Prior Art 
     A single-head plug includes poles (the above-mentioned center poles) which are disposed in plural positions in the axial direction of a single pole shaft, respectively. A multipolar connector is structured so that the body has a space (insertion space) into which a pole shaft of such a single-head plug is to be inserted. A plurality of contact pieces are incorporated into the insertion space. Generally, such a multipolar connector is well known as a jack type multipolar connector. A multipolar connector of this kind has been developed so as to be used in a portable telephone, a portable cassette tape recorder, a portable radio receiver, and the like. In order to correspond to the recent tendency to miniaturization of an apparatus, such a multipolar connector is requested to be microminiaturized. Furthermore, in order to cope with increased functions of an apparatus, a multipolar connector is requested to have a larger number of poles. 
     SUMMARY OF THE INVENTION 
     The present invention has been conducted in view of the above-mentioned circumstances. 
     It is an object of the invention to increase the number of poles by disposing contact pieces by using a boss portion which protrudes from the front face of the body. 
     It is another object of the invention to enable the number of poles of a multipolar connector to be increased while preventing the size of the multipolar connector as far as possible from being increased. 
     Referring to the drawings, the multipolar connector of the invention will be described. The reference numerals in the figures are used in this paragraph in order to facilitate the understanding of the invention, and the use of the reference numerals is not intended to restrict the contents of the invention to the illustrated embodiment. 
     In the multipolar connector of the invention, a single-head plug including: a pole shaft  91  in which center poles  92  to  95  are disposed in plural positions in an axial direction; and at least one peripheral pole  96  which is opposed to an outside of a root of the pole shaft  91  with being separated from the root of the pole shaft  91  by a distance is used as a counter connector  90 . 
     The multipolar connector of the invention has: a body  10  comprising an insertion space  14  for the pole shaft  91 ; a plurality of predetermined contact pieces  60 ,  70 , and  80  attached to the body  10  and comprising predetermined contacts  63 ,  73 , and  83  which are to be in contact with the center poles  92  to  94  other than the center pole in the root of the pole shaft  91  that is inserted into the insertion space  14 , respectively; and a cylindrical boss portion  30  which protrudes from a front end of the body  10 , and which forms an insertion/extraction port  37  communicating with the insertion space  14 . 
     In the multipolar connector of the invention, therefore, a given number of poles, for example, three poles or first to third poles are ensured by the predetermined contact pieces  60 ,  70 , and  80 , and a larger number of poles are ensured by additional contact pieces which are placed on outer and inner peripheral sides of the boss portion  30 . Furthermore, one or plural poles are ensured by an additional contact piece(s) which is placed on the outer peripheral side of the boss portion  30 , and one or plural poles are ensured by an additional contact piece(s) which is placed on the inner peripheral side of the boss portion  30 . 
     The multipolar connector of the invention has: a plate-like first additional contact piece  40  which is placed in a placement position for the first additional contact piece  40 , the placement position being formed in at least one position of the outer peripheral side of the boss portion  30 ; a first additional contact  42  which is disposed on the first additional contact piece  40 , and which is to be in contact with the peripheral pole  96  disposed in the counter connector  90 ; a second additional contact piece  50  which is placed in a placement position for the second additional contact piece  50 , the placement position being formed in at least one position of the inner peripheral side of the boss portion  30 ; and a second additional contact  58  which is disposed on the second additional contact piece  50 , and which is to be in contact with the center pole  95  in the root of the pole shaft  91  of the counter connector  90 . 
     According to the invention, therefore, the poles the number of which is equal to a sum of the number of the first additional contact piece(s)  40 , and that of the second additional contact piece(s)  50  are added to the poles which are ensured by the predetermined contact pieces  60 ,  70 , and  80 . Since the first additional contact piece  40  which is disposed by using the boss portion  30  has a plate-like shape, miniaturization can be attained more easily as compared with the case where the first additional contact piece  40  is formed into a ring-like or cylinder-like shape. Particularly, the first additional contact piece  40  can be formed at least one of the right and left sides of the boss portion  30 . According to this configuration, the first additional contact piece  40  and the first additional contact  42  do not protrude into the spaces above and below the upper and lower faces of the body  10 , and hence increase of the size due to an increased height does not occur in the multipolar connector. 
     Preferably, the boss portion  30  is formed into a cylindrical shape having a flat circle-cutaway portion  31  which is formed by cutting away a part of an outer side of a peripheral wall of the boss portion, and the circle-cutaway portion  31  functions as the placement position for the first additional contact piece  40 . According to this configuration, the placement position for the first additional contact piece  40  can be made closer to the center of the boss portion  30 , and this is useful to suppress increase of the size of the multipolar connector. 
     Preferably, the first additional contact piece  40  has a first mounting plate portion  41  which is fixed to the body  10 , and forward elongates from the first mounting plate portion  41  along an outer face of the circle-cutaway portion  31  of the boss portion  30 , to form a gap between the piece and the outer face of the circle-cutaway portion  31 . According to this configuration, even when the multipolar connector is miniaturized, the first additional contact piece  40  can be easily incorporated through the first mounting plate portion  41 . When a configuration in which the first mounting plate portion  41  is fixed to the body  10  under a state where the portion is inserted into a mounting groove  15  that rearward elongates from a front end face of the body  10  is employed, particularly, the attachment of the first mounting plate portion  41  of the first additional contact piece  40  can be performed by inserting the portion into the mounting groove  15  from the front side of the body  10 . 
     Preferably, the first additional contact  42  outward protrudes from an inner side of a circle-cutaway space which is formed by the circle-cutaway portion  31  of the boss portion  30 . According to this configuration, in order to cause the first additional contact  42  to be surely in contact with the peripheral pole  96  of the counter connector, it is requested only to insert the pole shaft  91  of the counter connector into the insertion space  14  of the body  10 . Therefore, the stability of the contact between the first additional contact  42  and the peripheral pole  96  of the counter connector is improved. The circle-cutaway space is a space which, assuming that an outer peripheral face of the boss portion  30  has a completely circular face, is inevitably formed between the circular face and the circle-cutaway portion  31  by forming the circle-cutaway portion  31 . 
     In the invention, preferably, a cutaway portion  34  in which a narrow portion  32  is located on a side of the front end of the boss portion  30  and a wide portion  33  continuous to the narrow portion  32  is located on a side of a rear end of the boss portion  30  is disposed in a position in a circumferential direction of the peripheral wall of the boss portion  30 , the second additional contact piece  50  which is formed into an annular shape is placed in the placement position for the second additional contact piece  50  that is annularly formed on the inner periphery side of the boss portion  30 , a gap is ensured between the second additional contact piece  50  and the inner peripheral face of the boss portion  30 , the second additional contact piece  50  is split in a position in a circumferential direction of an annular portion  51  and has a pair of outward tongue pieces  52  and  53  which are disposed in end portions that are opposed to each other in the split portion, respectively, and the outward tongue pieces  52  and  53  are inserted into the wide portion  33  of the cutaway portion  34 , and displaceable in the wide portion  33  in separating and approaching directions in accordance with expanding and contracting deformations of the annular portion  51 . When the annular portion  51  is pressingly expanded by the pole shaft  91  of the counter connector inserted into the annular portion  51 , preferably, the pair of outward tongue pieces  52  and  53  are displaced in the separating directions, whereby the pieces are respectively opposed in an axial direction of the boss portion  30  to step portions  35  in a boundary area between the narrow portion  32  and the wide portion  33  of the cutaway portion  34 . 
     According to the invention, when the pole shaft  91  of the counter connector is inserted into the annular portion  51  of the second additional contact piece  50 , the pair of outward tongue pieces  52  and  53  of the annular portion  51  of the second additional contact piece  50  are displaced in the separating directions in the wide portion  33  of the cutaway portion  34  on the side of the boss portion  30 , to enter the state where the pieces are opposed to the step portions  35  in the axial direction of the boss portion  30 . When the counter connector is pulled out, therefore, the outward tongue pieces  52  and  53  are engaged with the step portions  35  to prevent the second additional contact piece  50  from being pulled out together with the counter connector. 
     The multipolar connector of the invention may have a configuration in which five poles are formed by the three predetermined contact pieces  60 ,  70 , and  80 , the one first additional contact piece  40 , and the one second additional contact piece  50 . According to this configuration, it is possible to configure a multipolar connector having five poles. In this case, a configuration may be employed in which, among the three predetermined contact pieces  60 ,  70 , and  80 , the two predetermined contact pieces  60  and  70  are respectively placed on right and left sides of the insertion space, and the remaining one or the predetermined contact piece  80  is placed in front of one of the two predetermined contact pieces  60  and  70 , or the predetermined contact piece  60 . In this case, the three predetermined contact pieces  60 ,  70 , and  80  can be easily attached to the body  10 , and this is useful to suppress increase of the size of the body  10 . This function is remarkably exerted when a configuration is employed in which the remaining one predetermined contact piece  80  is formed into a folded shape with respect to a mounting plate portion  81  that is disposed continuously from the predetermined contact piece  80 , and the predetermined contact piece  80  and the mounting plate portion  81  are placed in a mounting groove  25  that is passed through the body  10  in a thickness direction. 
     As described above, according to the invention, the first and second additional contact pieces which can be used for increasing the number of poles are placed on the outer and inner peripheral sides of the boss portion, and hence the number of poles of the multipolar connector can be increased without thickening the connector. Since the first additional contact piece is formed into a plate-like shape, furthermore, there is an advantage that the first additional contact piece itself can be extremely miniaturized as compared with a contact piece of the prior art which has a ring-like or cylinder-like shape, and this is remarkably useful for miniaturizing the multipolar connector. In the configuration where the first additional contact piece is placed by using the circle-cutaway portion formed on the boss portion, the boss portion can be miniaturized and an excessive space due to the miniaturization of the boss portion is used as a space for placing the first additional contact piece, thereby producing an advantage that, although the number of poles is increased as a result of using the first additional contact piece, the size of the whole of the multipolar connector is not increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view of the multipolar connector of the invention as seeing obliquely from the front side; 
     FIG. 2 is a schematic perspective view showing a first additional contact piece and a first mounting plate portion; 
     FIG. 3 is a schematic perspective view of a predetermined contact piece; 
     FIG. 4A is a schematic perspective view of another predetermined contact piece; 
     FIG. 4B is a view as seeing in the direction of an arrow IVB of FIG. 4A; 
     FIG. 5 is a schematic perspective view of a further predetermined contact piece; 
     FIG. 6 is a schematic perspective view of a second additional contact piece; 
     FIG. 7A is a view as seeing in the direction of an arrow VIIA of FIG. 6; 
     FIG. 7B is a view as seeing in the direction of an arrow VIIB of FIG. 6; 
     FIG. 7C is a view as seeing in the direction of an arrow VIIC of FIG. 6; 
     FIG. 8 is a schematic horizontal section view of the multipolar connector shown in FIG. 1; 
     FIG. 9 is a schematic front view of a multipolar connector; 
     FIG. 10 is a schematic section view showing the case where a multipolar connector is perpendicularly cut in the vicinity of the front end of the body; 
     FIG. 11 is a schematic section view corresponding to FIG.  10  and showing a state where a pole shaft of a counter connector is inserted into an insertion space; and 
     FIG. 12 is a side view of the counter connector. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, an embodiment of the invention will be described. 
     As shown in FIG. 1, the body  10  of a multipolar connector A is formed into a rectangular parallelepiped of a low profile. The upper face  11  and the lower face  12  (see FIG. 9 or  10 ) of the body  10  are flat. The front face  13  of the body  10  is configured as a laterally prolonged flat face. A boss portion  30  which is integrated with the body  10  and made of a synthetic resin protrudes forward from a center portion of the front face  13 . 
     The boss portion  30  is formed into a cylindrical shape having a circle-cutaway portion  31  which is formed to be flat by cutting away a part of the outer side of the peripheral wall. The circle-cutaway portion  31  functions as a placement position for a first additional contact piece  40 . In the illustrated multipolar connector A, the diameter of the cylindrical boss portion  30  is larger than the distance (the height of the body  10 ) between the upper face  11  and the lower face  12  of the body  10 , and upper and lower portions of the boss portion  30  are partly cut away so as to be flat, with the result that the portions are substantially flush with the upper face  11  and the lower face  12  of the body  10 , respectively. Alternatively, upper and lower portions of the boss portion may be formed into an arcuate shape, and the arcuate portions may protrude from the upper and lower faces of the body  10 , respectively. 
     As shown in FIG. 1 or  9 , a cutaway portion  34  is formed in a position (in the embodiment, an apex portion) in the circumferential direction of the boss portion  30 . In the cutaway portion  34 , a narrow portion  32  is located on the side of the front end of the boss portion  30 , and a wide portion  33  continuous to the narrow portion  32  is located on the side of the rear end of the boss portion  30 . In the cutaway portion  34 , step portions  35  are disposed in a boundary area between the narrow portion  32  and the wide portion  33 . A notched hole  36  which elongates over the whole length of the boss portion  30  is opened in the left side of the boss portion  30 . 
     As shown in FIG. 8, an insertion space  14  into which a pole shaft  91  of a counter connector  90  (described later) is to be inserted is formed in the body  10 , and an insertion/extraction port  37  which communicates with the insertion space  14  is formed in the boss portion  30 . The insertion space  14  is opened in the rear end of the body  10 . 
     As shown in FIGS. 1,  9 , and  10 , a mounting groove  15  which elongates in the thickness direction of the body  10  is formed in the right front side of the body  10 , and a recess  16  which communicates with the mounting groove  15  is formed in the upper face  11  of the body  10 . As shown in FIG. 9 or  10 , a mounting groove  17  which is opened in the lower face of the body  10  and which upward elongates is formed in the left side in the vicinity of the front end of the body. As shown in FIG. 8, mounting grooves  18  and  19  which are opened in the rear end are formed in the left and right sides that are on both sides of the insertion space  14 , respectively. An engaging projection  21  (see FIG. 1) is formed in a position in the vicinity of the rear end of the upper face  11  of the body  10 . A similar engaging projection (not shown) is formed also in the lower face  12  of the body  10 . 
     As shown in FIG. 2, the first additional contact piece  40  is formed into a long and narrow plate piece-like shape. A spherical first additional contact  42  which outward protrudes is formed in a tip end portion of the first additional contact piece  40 . A first mounting plate portion  41  which has a plate piece-like shape is formed so as to be continuous to the first additional contact piece  40 . In the first mounting plate portion  41 , an engaging piece  43  is bendingly formed in the upper edge, and a soldering terminal  44  is bendingly formed in the lower edge. As known from FIG. 1 or  8 , the first additional contact piece  40  is fixed to the mounting groove  15  which is formed in the right side of the body  10 , by inserting the first mounting plate portion  41  into the mounting groove  15  from the front side. In this case, an engaging claw  45  (see FIG. 2) formed on the first mounting plate portion  41  is engaged with a wall of the mounting groove  15 , thereby preventing the first mounting plate portion from slipping off from the mounting groove  15 . Under this mounting state, as shown in FIG. 1 or  8 , the first additional contact piece  40  forward elongates along a flat outer face of the circle-cutaway portion  31  of the boss portion  30 , and a gap S 1  is formed between the piece and the flat outer face of the circle-cutaway portion  31 . The first additional contact  42  outward protrudes from the inner side of the circle-cutaway space which is formed by the circle-cutaway portion  31  of the boss portion  30 . The gap S 1  is useful for absorbing an inward displacement of the first additional contact piece  40 . As shown in FIG. 1 or  8 , the soldering terminal  44  disposed on the first mounting plate portion  41  laterally protrudes from the lower side of the body  10 . 
     When the first additional contact piece  40  is placed so as to forward elongate along the flat outer face of the circle-cutaway portion  31  of the boss portion  30  as described above, the first additional contact piece  40  can be placed so as to be close to the center of the boss portion  30 . 
     Referring to FIG. 1,  50  denotes a second additional contact piece. As shown in FIGS. 6,  7 A,  7 B, and  7 C, the second additional contact piece  50  has an annular portion  51 . The inner diameter of the annular portion  51  in a natural state is slightly smaller than the outer diameter of the root of the pole shaft  91  of the counter connector  90  shown in FIG.  11 . In the annular portion  51 , a position in the circumferential direction is split, and outward tongue pieces  52  and  53  are disposed in a bent state integrally with a pair of end portions that are opposed to each other in the split portion, respectively. In a stage before the second additional contact piece  50  is placed in the boss portion  30 , the pair of outward tongue pieces  52  and  53  are opposed to each other with being separated by a distance which allows the tongue pieces to be inserted into the narrow portion  32  of the cutaway portion  34 , even when the tongue pieces are not to be displaced in an approaching direction. A bulge portion  54  is formed in the left side of the annular portion  51 . A plate piece portion  55  outward protrudes from the rear end of the bulge portion  54 . A mounting plate portion  56  rearward elongates from the outer end of the plate piece portion  55 . An engaging claw  57  is disposed on the mounting plate portion  56 . The reference numeral  58  denotes a second additional contact which is disposed in each of two positions of the annular portion  51 , and  59  denotes a soldering terminal which is disposed continuously from the mounting plate portion  56 . 
     The second additional contact piece  50  is fitted into the inner peripheral side of the boss portion  30  from the front side. Specifically, as shown in FIG. 8, the mounting plate portion  56  is inserted into the mounting groove  17  which is formed in the body  10 , the bulge portion  54  of the annular portion  51  is slightly fitted into the notched hole  36  of the boss portion  30 , and the pair of outward tongue pieces  52  and  53  are slightly fitted into the narrow portion  32  of the cutaway portion  34  of the boss portion  30 . Under this state, the second additional contact piece  50  is rearward pushed to cause the annular portion  51  to be fitted into the boss portion  30 . As a result, the bulge portion  54  is completely fitted into the notched hole  36 , and the pair of outward tongue pieces  52  and  53  are completely fitted into the wide portion  33  of the cutaway portion  34 . Furthermore, the mounting plate portion  56  is pressed into the mounting groove  17 , and the engaging claw  57  is engaged with an engaging recess  24  of the body  10 . Therefore, the second additional contact piece  50  in which the annular portion  51  is fittingly placed on the inner peripheral side of the boss portion  30  is prevented from slipping off from the boss portion  30 , by the engagement of the engaging claw  57  and the engaging recess  24 . Under this mounting state, a gap S 2  shown in FIG. 9 is ensured between the annular portion  51  and the inner peripheral face of the boss portion  30 . The gap S 2  enables the diameter of the annular portion  51  to be increased. As shown in FIG. 8 or  9 , the soldering terminal  59  laterally protrudes from the lower side of the body  10 . 
     Under a state where the second additional contact piece  50  is incorporated into the boss portion  30  as described above, as shown in FIG. 1, the pair of outward tongue pieces  52  and  53  are separated from the wall faces of the wide portion  33  of the cutaway portion  34 , and opposed to the narrow portion  32 . When the pole shaft  91  of the counter connector  90  is inserted into the annular portion  51  under this state as shown in FIG. 11, the annular portion  51  is pressingly expanded by the pole shaft  91 . In accordance with the expansion, the pair of outward tongue pieces  52  and  53  shown in FIG. 1 are displaced in the separating directions in the wide portion  33  of the cutaway portion  34 , and the pieces are opposed in the axial direction of the boss portion  30  to the step portions  35  in the boundary area between the narrow portion  32  and the wide portion  33 . When the pole shaft  91  of the counter connector  90  is pulled out, therefore, the outward tongue pieces  52  and  53  are engaged with the step portions  35  to prevent the second additional contact piece from being pulled out together with the counter connector. 
     When the counter connector  90  is repeatedly inserted and extracted, the annular portion  51  of the second additional contact piece  50  is plastically deformed to slightly increase its diameter. Such plastic deformation causes the pair of outward tongue pieces  52  and  53  to be always opposed to the step portions  35 . 
     As shown in FIG. 8, the three predetermined contact pieces  60 ,  70 , and  80  are placed so as to face the insertion space  14  formed in the body  10 . Among the predetermined contact pieces, the two predetermined contact pieces  60  and  70  are respectively placed on the right and left sides of the insertion space  14 , and the remaining one or the predetermined contact piece  80  is placed in front of the predetermined contact piece  60 . 
     The predetermined contact pieces  60 ,  70 , and  80  are shown in FIGS. 3 to  5 , respectively. 
     In the predetermined contact piece  60  shown in FIG. 3, the front end portion is formed into a folded shape with respect to a mounting plate portion  61 . A predetermined contact  63  is disposed on the predetermined contact piece  60 , and a soldering terminal  62  is disposed continuously from the mounting plate portion  61 . A projecting piece  64  which is bendingly formed on the mounting plate portion  61  can prevent the predetermined contact piece  60  from being broken by excessive deformation. The predetermined contact piece  60  is attached to the body  10  by inserting the mounting plate portion  61  into the mounting groove  18  of the body  10  from the rear side. Under this mounting state, as shown in FIG. 8, the predetermined contact  63  of the predetermined contact piece  60  protrudes into the insertion space  14 . In the illustrated multipolar connector, a movable contact  65  is formed in a tip end portion of the predetermined contact piece  60 , and a stationary contact  66  serving as a counter contact of the movable contact  65  is disposed in a rear end portion of the body  10 . The soldering terminal  62  on the side of the predetermined contact piece  60 , and a soldering terminal  67  which is disposed continuously from the stationary contact  66  protrude rearward and laterally from the lower side of the body  10 . 
     In the predetermined contact piece  70  shown in FIG. 4, the front end portion is formed into a folded shape with respect to a mounting plate portion  71 . A predetermined contact  73  is disposed on the predetermined contact piece  70 , and a soldering terminal  72  is disposed continuously from the mounting plate portion  71 . A projecting piece  74  which is bendingly formed on the mounting plate portion  71  can prevent the predetermined contact piece  70  from being broken by excessive deformation. The predetermined contact piece  70  is attached to the body  10  by inserting the mounting plate portion  71  into the mounting groove  19  of the body  10  from the rear side. Under this mounting state, as shown in FIG. 8, the predetermined contact  73  of the predetermined contact piece  70  protrudes into the insertion space  14 . In the illustrated multipolar connector, a movable contact  75  is formed in a tip end portion of the predetermined contact piece  70 , and a stationary contact  76  serving as a counter contact of the movable contact  75  is disposed on the body  10 . The soldering terminal  72  on the side of the predetermined contact piece  70 , and a soldering terminal  76  which is disposed continuously from the stationary contact  76  protrude laterally from the lower side of the body  10 . 
     In the predetermined contact piece  80  shown in FIG. 5, the upper end portion is formed into a folded shape with respect to a mounting plate portion  81 . A predetermined contact  83  is disposed on an arcuately curved portion of the predetermined contact piece  70 , and a soldering terminal  82  is disposed continuously from the mounting plate portion  81 . As shown in FIG. 10, the predetermined contact piece  80  is attached to the body  10  by inserting from the lower side into the mounting groove  25  that is passed through the vicinity of the front end portion of the body  10  in the thickness direction. Under this mounting state, as shown in FIG. 8, the predetermined contact  83  protrudes into the insertion space  14 . The soldering terminal  82  on the side of the predetermined contact piece  80  protrudes laterally from the lower side of the body  10 . 
     In the counter connector  90 , as shown in FIG. 12, the pole shaft  91  includes four or first to fourth center poles  92 ,  93 ,  94 , and  95  which are arranged in the axial direction. The first center pole  92  corresponds to the one predetermined contact piece  60 , the second center pole  93  to the other one predetermined contact piece  70 , and the third center pole  94  to the further one predetermined contact piece  80 . The fourth center pole  95  which is in the root of the pole shaft  91  corresponds to the second additional contact piece  50 . The counter connector  90  has a cylindrical peripheral pole  96  which is concentric with the pole shaft  91  so as to be opposed to the root of the pole shaft  91 . The peripheral pole  96  corresponds to the first additional contact piece  40 . 
     In the above-described multipolar connector A, when the pole shaft  91  of the counter connector  90  is inserted into the insertion space  14  of the body  10  through the insertion/extraction port  37  (see FIG. 1 or  9 ) of the boss portion  30  as shown in FIG. 11, the first to third center poles  92 ,  93 , and  94  of the pole shaft  91  are elastically in contact with the predetermined contacts  63 ,  73 , and  83  of the three predetermined contact pieces  60 ,  70 , and  80 , respectively. Moreover, the first additional contact  42  of the first additional contact piece  40  is in contact with the peripheral pole  96  of the counter connector  90 , and the second additional contact  58  (see FIG. 9) of the second additional contact piece  50  is elastically in contact with the fourth center pole  95  which is located in the root of the pole shaft  91  of the counter connector  90 . As apparent from the above, in the illustrated multipolar connector A, three poles are ensured by the three predetermined contact pieces  60 ,  70 , and  80  which are incorporated into the insertion space  14  of the body  10 , one pole is ensured by the first additional contact piece  40  which is placed in the circle-cutaway portion  31  of the boss portion  30 , and a further one pole is ensured by the second additional contact piece  50  which is placed in the inner peripheral side of the boss portion  30 , with the result that five poles are ensured as a total. 
     The peripheral pole  96  of the counter connector  90  has a cylindrical shape. Even when the first additional contact piece  40  is formed into a long and narrow plate piece-like shape, therefore, the first additional contact  42  is always elastically in contact with the peripheral pole  96  as far as the pole shaft  91  of the counter connector  90  is inserted into the insertion space  14 . Since the first additional contact  42  is formed in the tip end portion of the first additional contact piece  40  which is elastic and has a plate piece-like shape, the contact state between the first additional contact  42  and the peripheral pole  96  is stable and has high reliability. 
     In the embodiment, the five-pole connector in which the five poles are formed by the three predetermined contact pieces  60 ,  70 , and  80 , the single first additional contact piece  40 , and the single second additional contact piece  50  has been described. When the first additional contact piece and the second additional contact piece are disposed at a number of two or more, it is possible to configure a multipolar connector having poles the number of which is larger than five. When two or more first additional contact pieces are to be disposed, a placement position for each of the first additional contact pieces is formed in plural positions in the circumferential direction of the boss portion. The placement positions may be formed by circle-cutaway portions which are formed by cutting away plural parts in the circumferential direction of the outer peripheral side of the boss portion. When two or more second additional contact pieces are to be disposed, a placement position for each of the second additional contact pieces is formed in plural positions in the circumferential direction of the inner peripheral side of the boss portion. In this case, it is preferable to form the second additional contact pieces into a plate piece-like shape which elongates in the longitudinal direction. 
     The entire disclosure of Japanese Patent Application No. 2000-034857 filed on Feb. 14, 2000 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.