Patent Publication Number: US-7588454-B2

Title: Connector device and locking structure

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a pair of connectors connectable with each other and to a locking construction. 
   2. Description of the Related Art 
   Japanese Unexamined Patent Publication No. 2005-183342 discloses a connector device with first and second connectors. The first connector has a lock arm and an engaging portion is disposed at the leading end of the lock arm. The second connector is formed with a lock projection. The connectors can be locked in a properly connected state by engaging the lock arm of the first connector with the lock projection of the second connector. The engaging portion moves onto the lock projection during connection of the connectors to deform the lock arm resiliently. However, the engaging portion moves beyond the lock projection when the connectors reach the properly connected state. As a result, the lock arm restores resiliently so that the engaging portion engages the locking projection to lock the two connectors in the properly connected state. 
   A specified clearance is defined between the engaging portion and the locking projection of the above-described connector to prevent the engaging portion and the locking projection from interfering with each other when the lock arm resiliently restores. Thus, there may be backlash between the locking projection and the engaging portion, when the connector is subjected to vibration or the like. As a result, abnormal noise may be produced or terminals may slide against each other and abrade. The clearance between the locking projection and the engaging portion may be enlarged due to creep deformation if the connector device is used in a high-temperature and high-vibration environment. Therefore, a countermeasure against backlash is needed. 
   Japanese Utility Model Publication No. H01-98485 discloses a locking structure that uses a spring for locking two members in a properly connected state. The spring has two resiliently displaceable legs and is vertically displaceably mounted on a housing of a connector. The housing is formed with passages that receive locking projections of a mating housing. The legs of the spring project into the passages when the housing is connected properly with the mating housing, and are located backward of the locking projections in the separating direction from the mating housing. Thus, the connectors are locked in a properly connected state by the engagement of the legs and the locking projections. The spring can be pressed down to retract the legs from the passages so that the locked state can be canceled for separating the two connectors. 
   The spring of the above-described connector normally is pressed down directly by fingertips during the unlocking operation. However, the spring is narrower than the fingertips and is difficult to press. 
   The mating connector may have a wall that impedes access to the spring by an operator&#39;s fingertips. Thus, the unlocking operation may require use of a jig or the like, thereby requiring additional labor and time. 
   The spring of the above-described connector may be displaced to cancel the locked state inadvertently, for example, if the connector is used in a high-vibration environment. However, the spring must be in displaceable to a certain extent to cancel the locked state. Thus, a countermeasure is needed. 
   The invention was developed in view of the above situation, and an object thereof is to provide a connector device having an improved operation. 
   SUMMARY OF THE INVENTION 
   The invention relates to a connector assembly comprising first and second connectors that have first and second housings respectively. The connectors can be connected by fitting the first and second housings together. The second housing is formed with passages for receiving engageable portions of the first housing when the connectors are connected. The connector assembly also includes a spring that is mounted on the second housing. The spring has two resiliently displaceable legs that project into the passages of the second housing. The engageable portions of the first housing move forward in the passages as the connectors are being connected. Thus, the engageable portions contact the legs of the spring and displace the legs resiliently. The engageable portions of the first housing receive the resilient restoring forces of the legs of the spring when the connectors reach a properly connected state for pushing the housings together. 
   The second housing preferably has a fitting space with at least one slanted surface that is inclined to reduce the width of the fitting space towards the rear. The slanted surface and the first housing preferably are set to contact at a position where a clearance is defined before the first and second connectors in a connecting direction. However, the resilient legs of the spring press the engageable portions and push the first housing in the connecting direction. Thus, even if the housings are deformed due to creep or the like, the resilient restoring forces of the legs of the spring move the first housing forward to hold the first and second connectors in a properly connected state. Therefore, backlash of the two connectors is prevented. 
   The first housing may have an inclined surface that contacts the slanted surface of the second housing. The inclined surface preferably is inclined substantially in conformity with the slanted surface. Thus, the two housings are held in surface contact for strongly preventing backlash. 
   The spring may be displaceable between an inserted position where the legs project into the passages and a retracted position where the legs are substantially retracted from the passages. The second housing and/or the legs may include guides inclined with respect to a displacing direction of the spring. Thus, the spring can be displaced smoothly. 
   The second housing may have a mounting portion for the spring, and peripherally extending walls may be provided before and after the mounting portion. Thus, the spring is mounted between the walls to reduce backlash of the spring. 
   One of the connectors may include at least one unlocking arm extending substantially in the connecting direction of the connectors. The unlocking arm preferably is inclinable like a seesaw. A pressing portion is defined at the front end of the unlocking arm in the connecting direction and is arranged for deflecting the spring. An unlocking portion is defined at the rear end of the unlocking arm and can be pressed for deflecting the unlocking arm and displacing the spring from the inserted position to the retracted position. Accordingly, the spring need not be operated directly with fingertips and operational efficiency is good. Further, the rear disposition of the unlocking portion enables efficient access to the unlocking portion regardless of the size or shape of the other connector. Therefore, a separate jig is not needed so that labor and time required for an unlocking operation can be saved. 
   A supporting arm may extend in the connecting direction for supporting the unlocking arm. One end of the supporting arm may be connected with the rear end of the unlocking arm and the opposite end of the supporting arm may be connected with one of the housings. 
   A support may be disposed between one of the housings and a central part of the unlocking arm in forward and backward directions. The support contacts the unlocking arm or the housing during deflection of the unlocking arm. Therefore, the strength of the connecting part between the unlocking arm and the housing can be reduced. In contrast, the connecting part would be enlarged to have sufficient strength if the connecting part of the unlocking arm and the housing was the entire support for a seesaw-like inclining movement. The rigidity of the connecting part then would have to be increased and a correspondingly large force would be required for pressing the unlocking portion. However, the rigidity of the connecting part of the one connector and the unlocking arm can be reduced according to the invention. Therefore, a smaller force is required for the unlocking operation and the unlocking operation can be performed easily. 
   The locking structure may comprise guiding tapers at entrances for the legs to the passages. The guiding tapers guide the engaging portions towards the passages by resilient restoring forces of the legs. Accordingly, the engaging portions are pressed against the guiding tapers and are guided into the passages by the resilient restoring forces of the legs when the unlocking portion is pressed, and the spring is displaced from the retracted position to the inserted position. Thus, the spring need not be set to the inserted position again when the connectors are reconnected and locked after an unlocking operation is performed. As a result, the locking operation can be performed easily. 
   The pressing portion may be shaped to be deflected towards a side in which the spring is pressed. Thus, the spring will not separate from the front end of the unlocking arm during the unlocking operation. 
   The spring is displaced from the inserted position to the retracted position by being displaced away from the housing. 
   The unlocking arm may include at least one bulge at a side of the spring substantially opposite to the pressing portion. The pressing portion of the unlocking arm displaces the spring away from the connector to cancel the locked state when the operable portion of the unlocking arm is pressed. However, the spring is held between the bulge and the pressing portion to prevent the spring from separating from the housing. 
   The bulge may be disposed to correspond with a substantially middle of the spring between the two legs. Accordingly, a widthwise central part of the spring between the legs is held between the bulge and the pressing portion. Thus, one side of the spring will not be lifted up from the pressing portion. 
   The connector assembly may include a movable member, such as a slider, as an alternative to the unlocking arm. The movable member may be displaceable substantially along the connecting direction and may be formed with at least one pressing surface disposed for sliding contact with the spring as the movable member is moved. The pressing surface preferably is inclined for pressing the spring to the retracted position as the movable member is moved. Accordingly, the spring need not be operated directly with fingertips and operational efficiency is good. Further, the locked state can be canceled without difficulty even if there is no space to place fingertips near the front end of the connector due to the shape of the mating connector. The unlocking operation is more efficient than connector assemblies that require the use of a jig or the like for unlocking. 
   A contact may project from the slider for contacting the mating connector after the spring is displaced to the retracted position. Accordingly, the connector can be pulled out in a separating direction while the contact is held in contact with the other mating connector. Thus, an operation of separating the connectors can be performed more easily as compared with the case where the connector is pulled out of the mating connector while being held by fingertips. 
   The contact may project in the connecting direction at a side of the spring substantially opposite to the connector. Accordingly, the spring cannot displace away from the connector and the spring will not detach from the connector. 
   The spring may be at least partly mounted in a substantially circumferentially extending groove in the connector. Thus, the longitudinal displacement of the spring can be prevented. 
   These and other objects, features and advantages of the present invention will become more apparent upon reading of the following detailed description of preferred embodiments and accompanying drawings. It should be understood that even though embodiments are separately described, single features thereof may be combined to additional embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal section showing a properly connected state of a connector device according to one embodiment. 
       FIG. 2  is an external perspective view of a female connector. 
       FIG. 3  is a plan view in section of the female connector. 
       FIG. 4  is a plan view in section showing the properly connected state of the two connectors. 
       FIG. 5  is a lateral section showing the properly connected state of the two connectors. 
       FIG. 6  is a lateral section of the female connector showing a state where a locked state is canceled. 
       FIG. 7  is a longitudinal section showing a state where the two connectors are separated. 
       FIG. 8  is a longitudinal section showing a connector assembly according to a further embodiment locked in a properly connected state. 
       FIG. 9  is a front perspective view of a female connector. 
       FIG. 10  is a rear perspective view of the female connector. 
       FIG. 11  is a plan view of the female connector. 
       FIG. 12  is a side view of the female connector. 
       FIG. 13  is a lateral section showing a state where the connector device is locked in the properly connected state. 
       FIG. 14  is a lateral section showing the female connector in an unlocked state. 
       FIG. 15  is a longitudinal section showing the connectors separated. 
       FIG. 16  is an external perspective view showing a female connector according to a still further embodiment when viewed from front. 
       FIG. 17  is a rear perspective view of the female connector when viewed from behind. 
       FIG. 18  is a plan view of the female connector. 
       FIG. 19  is a rear view of the female connector. 
       FIG. 20  is an external perspective view partly in section showing the female connector. 
       FIG. 21  is a longitudinal section of the female connector. 
       FIG. 22  is a longitudinal section showing a state where two connectors are locked by a locking structure according to a first embodiment. 
       FIG. 23  is a lateral section showing the state of  FIG. 22 . 
       FIG. 24  is a lateral section of a female housing. 
       FIG. 25  is a plan view of a slider. 
       FIG. 26  is a side view of the slider. 
       FIG. 27  is a front view of the slider. 
       FIG. 28  is a front view of the female housing having a spring mounted thereon. 
       FIG. 29  is a plan view in section of the pair of connectors when the slider is at a lockable position. 
       FIG. 30  is a plan view in section of the pair of connectors when the slider is at an unlocking position. 
       FIG. 31  is a longitudinal section showing a state where a locked state of the pair of connectors is canceled. 
       FIG. 32  is a lateral section showing the state of  FIG. 31 . 
       FIG. 33  is a longitudinal section showing a state where two connectors are locked by a locking structure according to a second embodiment. 
       FIG. 34  is a side view of a slider. 
       FIG. 35  is a front view of the slider. 
       FIG. 36  is a longitudinal section showing a state where a locked state of the pair of connectors is canceled. 
       FIG. 37  is a longitudinal section showing a state where a connector assembly according to one further embodiment is locked in a properly connected state. 
       FIG. 38  is an external perspective view of a female connector. 
       FIG. 39  is a plan view of the female connector. 
       FIG. 40  is a side view of the female connector. 
       FIG. 41  is a lateral section showing a state where two connectors are locked in a properly connected state. 
       FIG. 42  is a lateral section showing the female connector in an unlocked state. 
       FIG. 43  is a longitudinal section showing a state where the two connectors are separated. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A connector in accordance with a first embodiment of the invention is illustrated in  FIGS. 1 to 7  and includes a male connector  10  and a female connector  20  that are connectable with each other. Ends of the two connectors  10 ,  20  that are connected to one another are referred to herein as the front ends and reference is made to  FIG. 1  concerning upper and lower sides. 
   The male connector  10  includes a male housing  11  and male terminals  12  are held in the male housing  11 . The male housing  11  is made e.g. of synthetic resin and includes front and rear receptacles  13  and  14 , each of which is a wide substantially rectangular tube. The front receptacle  13  has an open front end and the rear receptacle  14  has an open rear end. The thickness of the front portion of the front receptacle  13  in inward and outward directions is less than the thickness of the rear portion of the front receptacle  13 . Engaging projections  16  project laterally out from the opposite left and right surfaces of the male housing  11 . 
   Male terminals  12  are provided side by side in the width direction and are held while penetrating a back wall  15  of the male housing  11  between the front and rear receptacles  13  and  14 . Thus, front and rear parts of the male terminals  12  project respectively into the front and rear receptacles  13  and  14 . 
   The female connector  20  includes a female housing  21  made e.g. of synthetic resin and connectable with the male housing  11 . The female housing  21  includes a terminal accommodating portion  23  that accommodates female terminals  22  and an outer tube  24  surrounds the outer circumferential surface of the terminal accommodating portion  23 . 
   The terminal accommodating portion  23  is a wide substantially rectangular column that can fit into the front receptacle  13  of the male housing  11 . Three cavities  25  are formed side by side in the terminal accommodating portion  23  and the female terminals  22  can be inserted into the cavities  25  from behind. 
   A connecting portion  26  extends unitarily out from the rear end of the terminal accommodating portion  23  and the outer tube  24  projects unitarily forward from the connecting portion  26  to a position substantially aligned with the front end of the terminal accommodating portion  23 . A fitting space  27  is defined between the outer tube  24  and the terminal accommodating portion  23  for receiving the front receptacle  13  of the male housing  11 . Backlash preventing ribs  49  project into the fitting space  27  from positions on the outer peripheral surface of the terminal accommodating portion  23  and the inner peripheral surface of the outer tube  24  (see  FIG. 6 ) for preventing backlash between the front receptacle  13  and the outer tube  24 . An opening  24 A is formed in the lower surface of the outer tube  24  and can receive a retainer  48  for retaining the female terminals  22  in the terminal accommodating portion  23 . 
   A seal ring  28  is fit at a position near the rear end of the outer peripheral surface of the terminal accommodating portion  23 . The seal ring  28  can be squeezed or deformed between the front receptacle  13  and the terminal accommodating portion  23  to provide sealing between the housings  11  and  12 . 
   Finger placing portions  29  project laterally out from the opposite left and right surfaces of the female housing  21  near the rear end of the female housing  21 . Fingertips can be placed on the finger placing portions  29  to pull the female housing  21  out for separating the connectors  10 ,  20 . 
   Two laterally spaced standing walls  31  project out from the upper surface of the outer tube  24 . The standing walls  31  extend continuously in forward and backward directions from the rear end of the female housing  21  to a position near the front end thereof. 
   First positioning walls  32  are provided on the opposite side surfaces of the outer tube  24  extend circumferentially from the front ends of the standing walls  31 . A second positioning wall  33  is provided on the outer tube  24  before the first positioning walls  32 . The second positioning wall  33  extends circumferential wall over the upper surface and the opposite side surfaces of the outer tube  24 . The first and second positioning walls  32  and  33  are substantially parallel to each other and are spaced in forward and backward directions. 
   A forwardly open front recess  34  is formed in a widthwise intermediate position of a part of the second positioning wall  33  and extends substantially along the upper surface of the outer tube  24 . The front recess  34  becomes gradually deeper from the opposite widthwise ends thereof towards the center thereof, and its depth in an intermediate part is less than ⅔, preferably about half the thickness of the second positioning wall  33  in forward and backward directions. 
   A mounting portion  35  is defined on the outer tube  24  between the first and second positioning walls  32  and  33 . The mounting portion  35  is formed by an inwardly recessed part of the outer peripheral surface of the outer tube  24  between the first positioning walls  32  and the second positioning wall  33 . 
   Passages  36  are formed in opposite left and right walls of the outer tube  24  for receiving the engaging projections  16  of the male housing  11 . Each passage  36  opens inwardly towards the terminal accommodating portion  23  and communicates with the mounting portion  35 . 
   Communicating portions  37  provide communication between the mounting portion  35  and the passages  36  and upper and lower surfaces  37 A and  37 B of communicating portions  37  are inclined to approach each other from the outer side towards the inner side. A vertical dimension of the communicating portions  37  at the inner sides is substantially equal to that of the engaging projections  16  (see  FIG. 5 ). 
   The connector further includes a U-shaped spring  50  with two legs  51  that extend down from opposite ends of an upper side  52 . The legs  51  are substantially normal to the upper side  52  in an undeflected state of the spring  50 . However, the legs  51  can be deflected resiliently away from one another. 
   Engaging portions  53  are bent to define inwardly pointed shapes at bottom parts of the legs  51 . The engaging portions  53  project into the passages  36  via the communicating portions  37 . Each engaging portion  53  includes an upper inclined side  53 A inclined in towards the other leg  51  and a lower inclined sides  53 B. Thus, the engaging portions  53  are shaped to have a smaller dimension in a vertical direction towards the inner side. The upper and lower inclined sides  53 A and  53 B have substantially the same inclinations as the upper and lower surfaces  37 A and  37 B of the communicating portions  37 . Boundaries between the upper and lower inclined sides  53 A and  53 B of the engaging portions  53  are rounded. 
   The spring  50  is mounted for displacement between an inserted position IP (see  FIG. 5 ) where the engaging portions  53  project into the passages  36  and a retracted position RP (see  FIG. 6 ) where the engaging portions  53  are retracted from the passages  36 . The engaging portions  53  projecting into the passages  36  are pressed by the engaging projections  16  passing the passages  36  when the spring  50  is at the inserted position IP. Thus, the legs  51  are deformed resiliently out. The legs  51  restore resiliently so that the engaging portions  53  engage engageable surfaces  16 B of the engaging projections  16  to lock the male housing  11  when the two connectors  10 ,  20  reach a properly connected state. 
   The spring  50  can be displaced up and away from the female housing  21  from the inserted position IP to the retracted position RP. The engaging portions  53  are retracted from the passages  36  to cancel the locked state when the spring  50  reaches the retracted position RP. More particularly, the engaging portions  53  that are retracted from the passages  36  are located at the outer sides of the upper surfaces  37 A of the communicating portions  37 . 
   A rear recess  38  is formed at a part of the upper surface of the outer tube  24  between the standing walls  31 . The rear recess  38  is in an intermediate position in forward and backward directions and extends to the rear edge of the female housing  21  (see  FIG. 7 ). The depth of the rear recess  38  gradually increases towards the back. 
   An unlocking arm  39  is provided on the outer tube  24  between the standing walls  31 . The unlocking arm  39  extends forward and backward along the connecting direction with the male housing  11 . Rotary shafts  41  extend in the width direction to connect the unlocking arm  39  with the standing walls  31  so that the unlocking arm  39  is inclinable like a seesaw. 
   A pressing portion  42  is defined near the front end of the unlocking arm  39  and is located below the spring  50  for pressing the spring  50  up and away from the female housing  21  when the unlocking arm  39  is inclined. A front wall  43  stands up at the front end of the unlocking arm  39 . The front wall  43  enters the front recess  34  of the second positioning wall  33 , and the rear surfaces of the front wall  43  and the second positioning wall  33  are substantially flush. The front wall  43  has a substantially mountain shape so that the width of the front wall  43  decreases gradually towards the upper end. 
   A hook  44  projects back near the upper end of the front wall  43 . A projecting distance of the hook  44  from the front wall  43  is substantially equal to or more than about half the dimension of the spring  50  in forward and backward directions. The hook  44  is above the upper side  52  of the spring  50 . A clearance is defined between the hook  44  and the pressing portion  42  and has a dimension substantially equal to the vertical dimension of the spring  50 . The upper side  52  of the spring  50  is held between the hook  44  and the pressing portion  42  with substantially no clearance in the vertical direction. The hook  44  is at a position corresponding to a widthwise central part of the spring  50  between the legs  51 . 
   The spring  50  is held substantially entirely between the first positioning walls  32  and the front wall  43 , the second positioning wall  33  with substantially no clearance defined in forward and backward directions. The heights of the first and second positioning walls  32  and the second positioning wall  33  are set so that the spring  50  is held therebetween over a displaceable range between the inserted position IP and the retracted position RP. 
   An unlocking portion  45  is provided near a rear end of the unlocking arm  39 . The lower surface of the unlocking portion  45  is inclined out and up towards the rear end to reduce the thickness of the unlocking portion  45 . The inclination of the lower surface of the unlocking portion  45  and the rear recess  38  of the female housing  21  ensures a sufficient downward displacement of the unlocking portion  45  while the height of the female housing  21  is suppressed. 
   The spring  50  is at the inserted position IP and the female housing  21  is in a state lockable into the male housing  11  when the unlocking arm  39  is in a natural state. A state of the unlocking arm  39  at this time is called a lockable state. The unlocking portion  45  can be pressed to incline the unlocking arm  39  so that the pressing portion  42  displaces the spring  50  up and out towards the retracted position RP to cancel the locked state with the male housing  11 . This state of the unlocking arm  39  is called an unlocking state. 
   The rear part of the fitting space  27  of the female connector  20  has slanted surfaces  46  that incline to approach the terminal accommodating portion  23  and to reduce the width and height of the fitting space  27 . The slanted surfaces  46  are formed on the inner peripheral surface of the outer tube  24  facing the terminal accommodating portion  23  and at a back end of the fitting space  27 . More specifically the slanted surfaces  46  are in a range of the inner peripheral surface of the outer tube  24  from a position immediately before the connecting portion  26  to a position near the front end of the seal ring  28 . Additionally, the slanted surfaces  46  extend around substantially the entire inner peripheral surface of the outer tube  24  (specifically, upper, lower, left and right surfaces excluding four corners) to reduce the dimensions of the fitting space  27  in vertical and transverse directions. The width at the back end of the fitting space  27  is less than the thickness of a front end part of the front receptacle  13  of the male housing  11 . Thus, the front receptacle  13  contacts the slanted surfaces  46  when the two connectors  10 ,  20  reach the properly connected state and before the front end of the front receptacle  13  reaches the back end of the fitting space  27 . Thus, a clearance is defined forward of the front receptacle  13 . 
   The front part of the front receptacle  13  is formed with inclined surfaces  17  that conform with the inclinations of the slanted surfaces  46 . The inclined surfaces  17  are formed around substantially the entire outer periphery of the front receptacle  13  and are inclined outwardly from the front end of the front receptacle  13  towards the rear end. Thus, the inclined surfaces gradually thin the front receptacle  13  towards the front. Parts of the inner peripheral surface of the front receptacle  13  corresponding to the inclined surfaces  17  are to be held in close contact with the seal ring  28 . 
   The engaging projections  16  of the male connector  10  are arranged at intermediate positions of the male housing  11  in forward and backward directions, and height positions that are substantially in the vertical center of the female housing  11 . A riding surface  16 A is defined at the front of each engaging projections  16  and is inclined moderately up from the front towards the rear so that the spring  50  can be moved easily onto the engaging projections  16 . An engageable surface  16 B is defined at the rear of each engaging projection  16  and is engageable with the spring  50 . 
   The engageable surfaces  16 B are inclined to reduce the projecting distance from the front receptacle  13  towards the rear. Hence the engageable surfaces  16 B produce forces for pushing the engaging projections  16  forward in the connecting direction upon receiving resilient restoring forces of the legs  51  in directions from the outer side toward the inner side of the front receptacle  13 . The inclination of the engageable surfaces  16 B is steeper than the inclination of the riding surfaces  16 A and is defined by the projecting distance of the engaging projections  16  with respect to a dimension smaller than the width of the spring  50  in forward and backward directions. The engageable surfaces  16 B face the communicating portions  37  when the two connectors  10 ,  20  reach the properly connected state. More specifically, the front edges of the engageable surfaces  16 B coincide with front edges of the communicating portions  37  and substantially the entire engageable surfaces  16 B are exposed to the space outside the outer tube  24  via the communicating portions  37  in the properly connected state of the connectors  10 ,  20 . The slanted surfaces  46  ensure the clearance before the front receptacle  13  and ensure that the engageable surfaces  16 B can be positioned easily with respect to forward and backward directions. For example, if the front end of the front receptacle contacted the back end of the fitting, the positions of the two housings with respect to forward and backward directions would be determined by such contact. However, the clearance is provided before the front receptacle  13  in this embodiment. Therefore, a slight error can be compensated by inserting the front receptacle  13  forward. 
   The unlocking arm  39  is set in the lockable state and the spring  50  is set at the inserted position IP prior to the start of a connecting operation. The female housing  21  then is gripped by fingers and gradually pushed into the male connector  10  while the passages  36  of the female housing  21  are positioned with respect to the engaging projections  16  of the male housing  11 . The engaging projections  16  then move forward in the passages  36  and the riding surfaces  16 A contact the engaging portions  53  of the spring  50 . The engaging portions  53  of the spring  50  move onto the engaging projections  16  and the legs  51  are displaced outwardly in directions away from each other as the engaging projections  16  move further forward. At this time, the hook  44  of the unlocking arm  39  is above the spring  50  and prevents the spring  50  from being lifted as the legs  51  displace resiliently. 
   The front end part of the front receptacle  13  of the male housing  11  then is inserted towards the back side of the fitting  27  while pressing and deforming the seal ring  28 . The inclined surfaces  17  of the front receptacle  13  contact the slanted surfaces  46  when the connectors  10 ,  20  reach the properly connected state to prevent further forward movement. The projecting end surfaces between the riding surfaces  16 A and the engageable surfaces  16 B of the engaging projections  16  pass the spring  50 . Thus, the legs  51  resiliently restored and contact the engageable surfaces  16 B arranged to face the communicating portions  37 . As a result, the two housings  11 ,  21  are locked into together. At this time, the engaging portions  53  of the legs  51  are pressed against the engageable surfaces  16 B at positions near the front end. In the properly connected state of the two connectors  10 ,  20 , the seal ring  28  and the rubber plugs  47  seal inner spaces of the two housings  11 ,  21  and the male and female terminals  12 ,  22  are properly electrically connected. 
   The inclined surfaces  17  of the male connector  10  and the slanted surfaces  46  of the female connector  20  are held in contact so that the clearance is before the male connector  10  when the connectors  10 ,  20  are connected properly. Additionally, the engaging projections  16  are pressed forward by resilient forces of the legs  51  to hold the connectors  10 ,  20  in close contact. The male connector  10  or the female connector  20  could be deformed due to creep or the like, and, for example, the outer tube portion  24  may be deformed outwardly. However, the male housing  11  is pushed forward by the resilient restoring forces of the legs  51  and a forward movement corresponding to this deformation is made to hold the two connectors  10 ,  20  in close contact. Therefore, backlash of the connectors  10 ,  20  is prevented even if the connectors  10 ,  20  are used in a high-temperature and/or high-vibration environment and fine sliding abrasion of the terminals  12 ,  22  can be avoided. 
   The close surface contact between the inclined surfaces  17  and the slanted surfaces  46  prevent backlash. In addition, the seal ring  28  is squeezed between the front receptacle  13  and the terminal accommodating portion  23  and exerts a resilient force to press the front receptacle  13  against the outer tube  24 . In this way, the slanted surfaces  46  and the inclined surfaces  17  are held in close contact over substantially their entire peripheries. 
   The spring  50  is mounted between the first and second positioning walls  32  and  33  to have its displacements in forward and/or backward directions prevented. Vertical displacements of the spring  50  are prevented by the hook  44  and the pressing portion  42 . As a result, the backlash between the spring  50  and the female housing  21  is prevented. 
   The two locked connectors  10 ,  20  can be separated if necessary by placing fingertips on the unlocking arm  39  in the lockable state and pressing the unlocking portion  45  down. Thus, the unlocking arm  39  inclines and the pressing portion  42  displaces the spring  50  up and out. The upper inclined sides  53 A of the engaging portions  53  are guided out along the upper surfaces  37 A of the communicating portions  37  as the spring  50  moves up. Thus, the legs  51  deform resiliently away from each other. The upper inclined sides  53 A and the upper surfaces  37 A of the communicating portions  37  are inclined. Therefore, the legs  51  are unlikely to get caught while being deformed and the spring  50  can be displaced smoothly. The engaging portions  53  are at the upper ends of the upper surfaces  37 A of the communicating portions  37  and are retracted from the passages  36  when the unlocking arm  39  reaches the unlocking state. Thus, the engaging projections  16  are freed from the locked state and can move backward. Accordingly, pressing the unlocking portions  45  displaces the spring  50  from the inserted position IP to the retracted position RP and cancels the locked state. Operational efficiency is good since the spring  50  need not be operated directly by fingertips. Fingers grip the finger placing portions  29  of the female housing  21  from the opposite sides and pull the female housing  21  backward after the locked state is canceled to separate the two connectors  10 ,  20 . 
   The fingers that had pressed the unlocking portion  45  are moved away after separating the connectors  10 ,  20 . As a result, the unlocking arm  39  naturally returns to the lockable state and the engaging portions  53  of the legs  51  are pressed against the upper surfaces  37 A of the communicating portions  37  with the resilient restoring forces. The engaging portions  53  then are guided into the passages  36  by the inclinations of the upper surfaces  37 A of the communicating portions  37  and, accordingly, the spring  50  is displaced from the retracted position RP to the inserted position IP. Thus, the spring  50  is set automatically to the inserted position IP merely by moving the fingertips away from the unlocking portion  45 . The spring  50  need not be set to the inserted position IP again upon reconnecting the two connectors  10 ,  20  and time and labor for the connecting and separating operations of the connectors  10 ,  20  is saved. 
   As described above, the male connector  10  and the slanted surfaces  46  of the female connector  20  contact while defining a clearance before the male connector  10  when the connectors  10 ,  20  reach the properly connected state. Additionally, the resiliently restored legs  51  press the engageable surfaces  16 B so that the male connector  10  is pressed forward towards the female connector  20 . Accordingly, even if the connectors  10 ,  20  are deformed due to creep or the like, the male housing  11  is pushed forward by the resilient restoring forces of the legs  51  and moves forward. Accordingly, the male connector  10  and the slanted surfaces  46  of the female connector  20  are held in contact. Therefore, backlash of the connectors  10 ,  20  is prevented. 
   The invention is not limited to the above described and illustrated embodiment. For example, the following modifications are also embraced by the technical scope of the present invention. 
   Although the spring  50  is mounted on the female connector  20  in the above embodiment, the invention is not limited to this and the spring may be mounted on the male connector while being held in such a state engageable with the female connector. At this time, the female connector may be provided with the engageable portions, the male connector may be formed with the passages, and the spring may be mounted, for example, on a rear end part of the male housing. Alternatively or additionally, the engageable portions may be provided on the inner side of the outer tube of the female housing, and the legs of the spring may contact the engageable portions to be displaced resiliently in upon connecting the two connectors, whereas the legs may be restored resiliently out to engage the engageable surfaces of the engaging portions when the two connectors reach the properly connected state. 
   Although the spring  50  reaches the retracted position RP by being displaced up and out from the inserted position IP in the above embodiment, it may reach the retracted position by being displaced down from the inserted position. 
   Although the locked state is canceled by operating the unlocking arm  39  in the above embodiment, the locked state may be canceled by directly displacing the spring with fingertips. 
   Although the upper surfaces  37 A of the communicating portions  37  and the upper inclined sides  53 A of the engaging portions  53  are both inclined in the above embodiment, only either ones thereof may be inclined. 
   A second embodiment of the invention is illustrated in  FIGS. 8 to 15  and relates to a locking structure for locking female and male connectors F and M together. Ends of the two connectors to be connected are referred to as front ends and reference is made to  FIG. 8  concerning upper and lower sides. 
   The male connector M includes a male housing  150  and male terminals  151  are held in the male housing  150 . The male housing  150  is made e.g. of synthetic resin and includes wide tubular front and rear receptacles  152  and  153 . The front receptacle  152  has an open front end and the rear receptacle  153  has an open rear end. Male terminals  151  are held while penetrating a back wall  154  between the front and rear receptacles  152  and  153  of the male housing  150 . Front and rear parts of the male terminals  151  project forward and backward in the front and rear receptacles  152  and  153 . 
   Engaging projections  155  project laterally out from the left and right surfaces of the male housing  150 . The engaging projections  155  are arranged at intermediate positions of the male housing  150  in forward and backward directions, and the height positions of the engaging projections  155  are located at a vertically intermediate position of the female housing  150 . The front surfaces of the engaging projections  155  are inclined moderately out from the front towards the rear so that the spring  140 , while engageable surfaces  155 A face rearward on the engaging projections  155 . 
   The female connector F includes a female housing  110  made e.g. of synthetic resin and connectable with the male housing  150 . The female housing  110  includes a terminal accommodating portion  111  that accommodates female terminals  134 . An outer tube  112  surrounds the outer peripheral surface of the terminal accommodating portion  111 . 
   The terminal accommodating portion  111  is a substantially rectangular column that can fit into the front receptacle  152  of the male housing  150  and has a wide cross section. Cavities  113  are formed substantially side by side in the terminal accommodating portion  111  and the female terminals  134  can be inserted into the cavities  113  from behind. 
   A connecting portion  114  unitarily connects the front end of the terminal accommodating portion  111  to the rear end of the outer tube  112 , and the outer tube  112  extends forward from the connecting portion  114 . A clearance is defined between the outer tube  112  and the rear end of the outer tube  112  for receiving the front receptacle  152  of the male housing  150 . A seal ring  115  is fit on a rear part of the outer peripheral surface of the terminal accommodating portion  111 . The seal ring  115  is dimensioned to be squeezed between the front receptacle  152  and the terminal accommodating portion  111  to provide sealing between the housings. An opening  112 A is formed in the lower surface of the outer tube  112  to assemble a retainer  135  for retaining the female terminals  134  in the terminal accommodating portion  111 . 
   Finger placing portions  116  project sideways from the opposite left and right surfaces near a rear end of the female housing  110 . Fingertips can be placed on the finger placing portions  116  to pull the female housing  110  for separating the connectors F, M. 
   Standing walls  117  are provided on the upper surface of the outer tube  112  and extend in forward and backward directions. The standing walls  117  extend continuously from the rear end of the female housing  110  to a position near the front end. An unlocking arm  125  is provided between the standing walls  117 . 
   A rear recess  118  is formed in a part of the upper surface of the outer tube  112  between the standing walls  117  and extends from an intermediate position in forward and backward directions to the rear edge. The depth of the rear recess  118  is increased gradually towards the back side. 
   First positioning walls  119  are provided on the opposite left and right surfaces of the outer tube  112 . The first positioning walls  119  extend substantially circumferentially along the opposite side surfaces of the outer tube  112  from the front ends of the standing walls  117 . A second positioning wall  121  is provided on the outer tube  112  before the first positioning walls  119 . The second positioning wall  121  extends substantially circumferentially over the upper surface and the opposite side surfaces of the outer tube  112 . The second positioning wall  121  and the first positioning walls  119  are substantially parallel to each other, and are spaced in forward and backward directions. 
   A front recess  122  is recessed in a widthwise intermediate position of the second positioning wall  121  and becomes gradually deeper from the opposite widthwise sides towards the center. The front recess  122  has a depth in an intermediate part of less than about ⅔, more preferably about half the thickness in forward and backward directions of the second positioning wall  121  (see  FIG. 11 ). 
   A mounting portion  123  is defined on a part of the outer tube  112  between the first positioning walls  119  and the second positioning wall  121 . The mounting portion  123  is formed by an inwardly recessed part of the outer peripheral surface of the outer tube  112  between the first positioning walls  119  and the second positioning wall  121 . 
   Passages  133  are formed in the opposite left and right walls of the outer tube  112 . Each passage  133  extends back from the front end of the outer tube  112  and communicates with the mounting portion  123 . 
   Upper surfaces  124 A and lower surfaces  124 B of communicating portions  124  provide communication between the mounting portion  123  and the passages  133 , and are inclined to approach each other from the mounting portion  123  side towards the inner side. A vertical dimension of the communicating portions  124  at the inner sides is substantially equal to a vertical dimension of the engaging projections  155  (see  FIG. 13 ). The upper surfaces  124 A of the communicating portions  124  are inclined from the mounting portion  123  towards the passages  133 . 
   The connector also includes a substantially U-shaped spring  140  with two legs  141  connected at their top ends by an upper member  142 . the legs  141  are resiliently displaceable towards and away from one another. The spring  140  can be mounted on the mounting portion  123  of the female housing  110  while the legs  141  extend substantially vertically down along a moving direction of the spring  140  with respect to the upper member  142 . 
   Engaging portions  143  are defined at bottom ends of the legs  141  and are bent to have triangular shapes pointed inwardly and towards each other. These bent engaging portions  143  project into the passages  133  when the spring  140  is mounted. The engaging portions  143  include upper inclined sides  143 A that incline towards each other and lower inclined sides  143 B that incline away from one another. The lower inclined sides  143 B have substantially the same inclinations as the lower surfaces  124 B of the communicating portions  124 , and the upper inclined sides  143 A have substantially the same inclinations as the upper surfaces  124 A of the communicating portions  124 . Boundaries between the upper inclined sides  143 A and the lower inclined sides  143 B of the engaging portions  143  are rounded. 
   The spring  140  is so mounted for displacement between an inserted position IP (see  FIG. 13 ) where the engaging portions  143  project into the passages  133  and a retracted position RP (see  FIG. 14 ) where the engaging portions  143  are retracted from the passages  133 . The engaging portions  143  project into the passages  133  when the spring  140  is at the inserted position IP and are pressed by the engaging projections  155  as the engaging projections  155  pass the passages  133 . Thus, the legs  141  deform resiliently out. The legs  141  restore resiliently when the connectors F, M reach a properly connected state, so that the engaging portions  143  engage the engageable surfaces  155 A to lock the male housing  150 . The spring  140  can be displaced outwardly and away from the female housing  110  from the inserted position IP to the retracted position RP. The engaging portions  143  are retracted from the passages  133  when the spring  140  reaches the retracted position RP to cancel the locked state. The engaging portions  143  that are retracted from the passages  133  are at the outer sides of the upper surfaces  124 A of the communicating portions  124 . The heights of the first and second positioning walls  119 ,  122  are set to be located before and after the spring  140  over a displaceable range of the spring  140  between the inserted position IP and the retracted position RP. 
   An unlocking arm  125  extends in forward and backward directions between the standing walls  117  and is connected unitarily to the standing walls  117  by rotary shafts  126  so that the unlocking arm  125  is above the outer tube  112 . The rotary shafts  126  extend slightly obliquely forward from the side surfaces of the unlocking arm  125  to the standing walls  117 . The unlocking arm  125  extends substantially from the rear end of the female housing  110  to the front recess  122  of the second positioning wall  121  and the rotary shafts  126  are at a substantially lengthwise intermediate position of the unlocking arm  125  (see  FIG. 11 ). The rotary shafts  126  permit resilient pivotable movement of the unlocking arm  125  like a seesaw about the rotary shafts  126 . However, the unlocking arm  125  is substantially parallel to the upper surface of the outer tube  112  (see  FIG. 8 ) when the unlocking arm  125  is in the natural state. 
   A pressing portion  127  is defined near the front end of the unlocking arm  125  and is below the spring  140 . Thus, the pressing portion  127  presses the spring  140  up and away from the female housing  110  when the unlocking arm  125  is inclined. A front wall  128  projects up at the front end of the unlocking arm  125 , and the pressing portion  127  is urged down by the front wall  128  as the spring  140  is moved up. The front wall  128  enters the front recess  122  of the second positioning wall  121 , and the rear surfaces of the front wall  128  and the second positioning wall  121  are substantially flush when the unlocking arm  125  is in a natural state. The front wall  128  has a substantially mountain or converging shape that gradually narrows towards the upper end. 
   A hook  129  projects back from the upper end of the front wall  128  and aligns with a substantially widthwise central position of the upper member  142  of the spring  140 . A projecting distance of the hook  129  preferably is less than about ⅔ of the spring  140 , more preferably substantially equal to half the width of the spring  140  in forward and backward directions. A clearance is defined between the hook  129  and the pressing portion  127  and has a dimension substantially equal to the vertical width of the spring  140 . Thus, the upper side member  142  of the spring  140  is held between the hook  129  and the pressing portion  127  with substantially no clearance defined in the vertical direction. The spring  140  is held between the first positioning walls  119 , the front wall  128  and the second positioning wall  121  with substantially no clearance in forward and backward directions. 
   An unlocking portion  131  is at a rear end of the unlocking arm  125  and can be pressed down to displace the pressing portion  127  up. The unlocking portion  131  is wider than a front part of the unlocking arm  125  before the rotary shafts  126  (see  FIG. 11 ), and a pressable slant  132  is inclined up towards the rear on the upper surface of the unlocking portion  131  (see  FIG. 8 ). The lower surface of the unlocking portion  131  inclines up towards the rear so that the unlocking portion  131  is thinned vertically towards the rear. The inclination of the lower surface of the unlocking portion  131  and the rear recess  118  of the female housing  110  ensure sufficient downward displacement of the unlocking portion  131  without increasing the height of the female housing  110 . 
   The spring  140  is at the inserted position IP when the unlocking arm  125  is in the natural undeflected state and the female housing  110  is locked with the female housing  150 . This state of the unlocking arm  125  is called a lockable state. The pressing portion  127  presses the spring  140  up when the unlocking portion  131  is pressed down to incline the unlocking arm  125  and to displace the spring  140  to the retracted position RP for canceling the locked state with the male housing  150 . This state of the unlocking arm  125  is called an unlocking state (see  FIGS. 14 and 15 ). 
   The unlocking arm  125  is in the lockable state, and the spring  140  is at the inserted position IP prior to connecting the male and female connectors M and F. The female housing  110  then is gripped by fingers and pushed gradually into the male connector M while the passages  133  of the female housing  110  are positioned with respect to the engaging projections  155  of the male housing  150 . Thus, the engaging projections  155  move forward in the passages  133  and the front surfaces of the engaging projections  155  contact the engaging portions  143  of the spring  140 . The engaging portions  143  of the spring  140  move onto the engaging projections  155  as the engaging projections  155  move farther forward and the legs  141  displace resiliently out and away from each other. The hook  129  of the unlocking arm  125  is above the spring  140  and prevents the spring  140  from being lifted up as the legs  141  deflect. 
   The projecting end surfaces of the engaging projections  155  pass the spring  140  as the two connectors F, M reach the properly connected state and the legs  141  resiliently restore. Thus, the engaging portions  143  and the engageable surfaces  155 A of the engaging projections  155  face each other in forward and backward directions to lock the two housings  110 ,  150  together. The seal ring  115  and the rubber plugs  136  for wires seal inner spaces of the two housings  110 ,  150  in the properly connected state of the two connectors F, M, and the male and female terminals  134 ,  151  are connected electrically. 
   The two connectors F, M may have to be separated. Therefore, fingertips are placed on the pressable slant  132  of the unlocking arm  125  in the lockable state to press the unlocking portion  131  down. The rotary shafts  126  rotate and function as supports so that the unlocking arm  125  is inclined and the pressing portion  127  presses the spring  140  up towards the retracted position RP. The engaging portions  143  are guided out along the inclinations of the upper surfaces  124 A of the communicating portions  124  and the legs  141  are resiliently deformed away from each other. The engaging portions  143  are at the upper ends of the upper surfaces  124 A of the communicating portions  124  and are retracted from the passages  133  when the unlocking arm  125  reaches the unlocking state. Thus, the engaging projections  155  are freed from the locked state and can move back. The fingers that had been placed on the finger placing portions  116  of the female housing  110  hold the female housing  110  from opposite sides and pull back to separate the two connectors F, M. 
   The spring  140  is displaced from the inserted position IP to the retracted position RP to cancel the locked state by pressing the unlocking portions  131 . Thus, the spring  140  need not be operated directly by fingertips, and operability is good. Further, the unlocking portion  131  is at the rear end of the unlocking arm  125 . Thus, the locked state can be canceled without any problem even if a male housing has a flange or the like projecting radially out at the front end. Therefore, labor and time is saved as compared with the case where unlocking requires a jig or the like as in the prior art. 
   The fingers that press the unlocking portion  131  can be moved away after separating the two connectors F, M. As a result, the unlocking arm  125  resiliently returns to release a force for displacing the spring  140  down and the resilient restoring forces of the legs  141  press the engaging portions  143  against the upper surfaces  124 A of the communicating portions  124 . The engaging portions  143  then are guided into the passages  133  by the inclinations of the upper surfaces  124 A of the communicating portions  124  and, accordingly, the spring  140  is displaced from the retracted position RP to the inserted position IP. Thus, the spring  140  is set automatically to the inserted position IP merely by moving the fingertips away from the unlocking portion  131 . Therefore, the spring  140  need not be set to the inserted position IP again upon reconnecting the two connectors F, M and labor required for locking the connectors F, M again in the connected state can be saved. 
   The pressing portion  127  is deflected towards the side towards which the spring  140  is pressed. Thus, the spring  140  will not separate from the unlocking arm  125  during the unlocking operation and the unlocking operation can be performed reliably. 
   The hook portion  129  of the unlocking arm  125  is arranged above the upper member  142  of the spring  140 . Thus, the hook  129  prevents upward displacement and separation of the spring  140 . 
   The pressing portion  127  of the unlocking arm  125  presses the widthwise central part of the spring  140  between the legs  141 . Thus, the force for pressing the spring  140  acts in a well-balanced manner and the unlocking operation is performed efficiently. 
   The spring  140  is displaced from the inserted position IP to the retracted position RP by pressing the unlocking portion  131 . Thus, the spring  140  need not be pressed directly by the fingertips to cancel the locked state. 
   A locking structure according to a further embodiment of the invention is illustrated in  FIGS. 16 to 21 . The locking structure of this embodiment differs from the previous embodiment in the shape of an unlocking arm  170 . Elements with same construction as the previous embodiment are described again and merely are identified by the same reference numerals. 
   This locking structure locks female and male connectors F, M together by at least one spring  140  mounted on a female housing  110 . Similar to the previous embodiment, the spring  140  is displaceable between an inserted position IP and a retracted position RP and reaches the retracted position RP by being displaced up and out from the inserted position IP. 
   The unlocking arm  170  is between two standing walls  117  provided on the upper surface of the female housing  110  and can be inclined like a seesaw. More particularly, the unlocking arm  170  is a substantially rectangular plate that is long in forward and backward directions and extends from the rear end of the female housing  110  to a front recess  122  of a second positioning wall  121 , similar to the previous embodiment. Additionally, the unlocking arm  170  is arranged substantially at an intermediate position between the standing walls  117  and is connected to the standing walls  117  by supporting arms  171  provided at the opposite sides. 
   The supporting arms  171  are long and narrow in an extending direction of the unlocking arm  170  and are provided for supporting the unlocking arm  170  in a state lifted up from the female housing  110 . The supporting arms extend substantially in forward and backward directions at intermediate positions between the unlocking arm  170  and the standing walls  117 . Substantially uniform clearances are defined at the opposite sides of each supporting arm  171 . Front end portions of the supporting arms  171  are connected with front parts of the standing walls  117  at positions near the first positioning walls  119 , and rear end portions of the standing portions  117  are connected with a rear part of the unlocking arm  170  near the unlocking portion  131 . Rotary shafts  172  are defined near front ends of the supporting arms  171  and the standing walls  117  and rotate when the unlocking arm  170  is inclined. 
   Similar to the previous embodiment, a pressing portion  127  is defined near a front end of the unlocking arm  170  for pressing the spring  140  up and out away from the female housing  110  when the unlocking arm  170  is inclined. A front wall  128  and a hook  129  are provided near the front end of the pressing portion  127 . An unlocking portion  131  is provided at the rear end of the unlocking arm  170  and can be pressed down to displace the pressing portion  127  up. A pressable slant  132  is formed on the upper surface of the unlocking portion  131  and the lower surface of the unlocking portion  131  is inclined up and out towards the rear end. 
   Two supports  173  project up from a part of the upper surface of the outer tube  112  between the standing walls  117 . The supports  173  are directly below a longitudinal central position of the unlocking arm  170  and before a rear recess  118 . Additionally, the supports  173  are spaced apart in the width direction of the unlocking arm  170  and have a mountain-shaped cross section so that the projecting distance gradually increases from the front and rear ends. The projecting distances of the supports  173  are set so that the projecting ends are distanced down from the lower surface of the unlocking arm  170  when the unlocking arm  170  is in a natural or locking state parallel to the upper surface of the outer tube  112 . However, the supports  173  contact the unlocking arm  170  when the unlocking portion  131  is pressed down to incline the unlocking arm  170  into an unlocking state. Thus, the supports  173  support and limit an inclining movement of the unlocking arm  170   
   An operation of locking the male connector M and the female connector F in a properly connected state is performed similar to the previous embodiment. More particularly, fingertips are placed on the pressable slant  132  of the unlocking arm  170  in the unlocking state to displace the unlocking portion  131  down. The rotary shafts  172  rotate to incline the supporting arms  171  and to bring the rear ends of the supporting arms  171  down. The entire unlocking arm  170  is displaced down towards the outer tube  112  as the supporting arms  171  are inclined, and the lower surface of the unlocking arm  170  contact the supports  173 . The unlocking portion  131  can be pressed further to incline the unlocking arm  170  about the supports  173 . As a result, the pressing portion  127  presses the spring  140  up to cancel the locked state, similar to the previous embodiment. The female housing  110  can be pulled back to separate the two connectors F, M after the locked state is canceled. 
   As described above, the locked state is canceled by pressing the unlocking portion  131  similar to the previous embodiment. Therefore the unlocking operation can be performed easily. 
   This unlocking arm  170  inclines like a seesaw with the supports  173  provided separately from the rotary shafts  172 . The supports  173  function as the supporting points for the inclining movement of the unlocking arm  170  and have strength for sustaining the force for pressing the pressing portion  127  up and for displacing the spring up towards the retracted position RP. The rotary shafts would have to be enlarged to have sufficient strength if the rotary shafts doubled as the supports. The rigidities of the rotary shafts then would have to be increased and a correspondingly larger force would be required for pressing the unlocking portion. However, the rigidities of the rotary shafts  172  can be reduced by separately providing the supports  173  in addition to the connecting rotary shafts  172 . Thus, the force required to press the unlocking portion  131  can be reduced, and the unlocking operation can be performed easily. 
   The invention is not limited to the above described and illustrated embodiments. For example, the following modifications are also embraced by the technical scope of the present invention. 
   Although the invention is applied to the locking structure of the connector device in the above embodiments, it can be applied to a locking structure for a pair of any members connectable with each other without being limited to the above. 
   The unlocking arm  125  is made inclinable like a seesaw by the rotatable connecting rotary shafts  126 . However, the unlocking arm may be in any mode that is inclinable like a seesaw. For example, a supporting leg standing up from the upper surface of the female housing may be provided and the unlocking arm may be inclined with this supporting leg as a support. 
   The supports  173  are provided on the outer tube  112 . However, the supports may be provided on the unlocking arm and may function as supporting points by contacting the outer tube when the unlocking portion is pressed to displace the unlocking arm down. 
   A further embodiment of the invention is illustrated in  FIGS. 22 to 32 . In this embodiment, a locking structure locks female and male connectors F, M that have been connected properly with each other. Ends of the two connectors F, M to be connected are referred to herein as front ends and reference is made to  FIG. 22  concerning upper and lower sides. 
   The male connector M is connected directly with a device, such as an automotive junction box, an instrument panel or the like. The male connector M has a male housing  250  with a front receptacle  252 A that projects forward from an outer wall  251  of the device and a rear receptacle  252 B that projects backward. The two receptacles  252 A,  252 B are substantially rectangular tubes. The front receptacle  252 A has an open front end and the rear receptacle  252 B having an open rear end. Male terminals  254  penetrate a back wall  253  between the front and rear receptacles  252 A and  252 B of the male housing  250 . Front and rear parts of the male terminals  254  project respectively forward and backward in the front and rear receptacles  252 A and  252 B. The outer wall  251  of the device projects from the outer peripheral surface of the female housing  210  over substantially the entire periphery. 
   Engaging projections  255  project laterally from the left and right surfaces of the male housing  250 . The engaging projections  255  are at intermediate positions of the male housing  250  in forward and backward directions, and are at substantially the vertical center of the female housing  250 . Front surfaces of the engaging projections  255  are inclined moderately up from the front towards the rear and engaging surfaces  255 A are defined at the rear ends of the engaging projections  255 . 
   The female connector F includes a female housing  210  made e.g. of synthetic resin and connectable with the male housing  250 . The female housing  210  includes a terminal accommodating portion  212  for accommodating female terminals  211  and an outer tube  213  surrounding the outer peripheral surface of the terminal accommodating portion  212 . 
   The terminal accommodating portion  212  is a substantially rectangular column that can fit into the front receptacle  252 A of the male housing  250  and has a slightly vertically longer cross section. Cavities  214  are formed in the terminal accommodating portion  212  and the female terminals  211  are insertable into the cavities  214  from behind. 
   The outer tube  213  extends substantially from the rear end to the front end of the terminal accommodating portion  212 , and a connecting portion  215  unitarily joins the rear end of the outer tube  213  to the rear end of the terminal accommodating portion  212 . 
   A clearance is defined between the outer tube  213  and the terminal accommodating portion  212  for receiving the front receptacle  252 A of the male housing  250 . A seal ring  216  is mounted in a rear position on the terminal accommodating portion  212  and is squeezed between the front receptacle  252 A of the male housing  250  and the terminal accommodating portion  212  of the female housing  210  to provide sealing. 
   A finger placing portion  217  projects out at a rear end of the female housing  210 . Fingers can be placed on the finger placing portion  217  to pull the female housing  210  out for separating the connectors F, M. 
   Standing walls  218  are provided on the upper surface of the outer tube  213  and extend in forward and backward directions. Guides  219  are provided on the opposed facing inner side surfaces of the standing walls  218 . The guides  219  preferably are substantially flat plates that project substantially perpendicularly from the inner side surfaces of the standing walls  218  (see  FIG. 24 ). The guides  219  are provided at vertical intermediate positions of the standing walls  218  and extend substantially continuously in forward and backward directions along the standing walls  218 . 
   A long narrow rectangular engaging window  221  is formed at an intermediate position of each guide  219  in forward and backward directions. The engaging window  221  is at a position on the respective guide  219  near the standing wall  218  and penetrates the guides  219  vertically. 
   Escaping recesses  222  are formed in parts of the upper surface of the outer tube  213  substantially facing the guides  219  (see  FIG. 22 ). The escaping recesses  222  extend from the rear edge of the female housing  210  to positions slightly before the front edges of the engaging windows  221 . The depth of each escaping recess  222  is reduced gradually from a part facing the engaging window  221  towards the front. Backlash preventing ribs  223  are provided on a part of the upper surface of the outer tube  213  between the standing walls  218  and extend in substantially forward and backward directions (see  FIG. 24 ). 
   First positioning walls  218 A are provided on the left and right surfaces of the outer tube  213  and extend circumferentially along the opposite side surfaces of the outer tube  213  from the front ends of the standing walls  218 . A second positioning wall  228  extends circumferentially on the outer tube  213  before the first positioning walls  218 A. The second positioning wall  228  and the first positioning walls  218 A are substantially parallel to each other and are spaced apart in forward and backward directions. 
   A wide rectangular insertion hole  228 A penetrates a widthwise central part of the second positioning wall  228  in forward and backward directions (see  FIG. 28 ) and a lift-preventing portion  227  is defined at an upper edge of the second positioning wall  228  above the insertion hole  228 A. 
   An inwardly recessed mounting portion  224  is defined on a part of the outer tube  213  between the first positioning walls  218 A and the second positioning wall  228 . 
   Passages  225  are formed in the opposite left and right walls of the outer tube  213  and receive the engaging projections  255  of the male housing  250 . Each passage  225  extends back from the front end of the outer tube  213  and communicates with the mounting portion  224 . 
   Upper and lower surfaces  226 A and  226 B of communicating portions  226  between the mounting portion  224  and the passages  225  are inclined to approach each other from the outer side towards the inner side. A vertical dimension of the communicating portions  226  at the inner sides is substantially equal to the vertical dimension of the engaging projections  255  (see  FIG. 23 ). 
   A spring  230  is mounted on the mounting portion  224 . The spring  230  has a substantially U-shape with an open bottom. Two legs  231  extend vertically down from opposite ends of an upper member  232  and are resiliently displaceable in directions substantially facing each other. 
   Bottom parts of the legs  231  are bent to have converging triangular engaging portions  233  that project into the passages  225 . The engaging portions  233  include upper inclined sides  233 A that incline in towards each other and lower inclined sides  233 B. The engaging portions  233  have smaller extensions in a vertical direction towards the inner side. Projecting ends between the upper and lower inclined sides  233 A and  233 B of the engaging portions  233  are rounded. 
   The spring  230  is displaceable between an inserted position IP (see  FIG. 23 ) where the engaging portions  233  project into the passages  225  and a retracted position RP (see  FIG. 22 ) where the engaging portions  233  are retracted from the passages  225 . The engaging portions  233  project into the passages  225  when the spring  230  is at the inserted position IP and are pressed by the engaging projections  255  passing the passages  225  deform the legs  231  outwardly. The legs  231  resiliently restore when the housings  210 ,  250  reach a properly connected state. As a result, engage the engaging portions  233  engage the respective engaging surfaces  255 A to lock the male housing  250 . The spring  230  reaches the retracted position RP by being displaced down from the inserted position IP towards the female housing  210 . The engaging portions  233  are retracted from the passages  225  when the spring  230  reaches the retracted position RP and are at the outer sides of the lower surfaces  226 B of the communicating portions  226  to cancel the locked state. 
   A slider  240  is assembled with the female housing  210  and is slidable in forward and backward directions along the connecting direction with the male housing  250 . The slider  240  has a main body  241  that is long in forward and backward directions and guiding ribs  242  are provided on the opposite sides of the main body  241  (see  FIG. 25 ). 
   The main body  241  is a substantially rectangular plate and has a length substantially equal to a distance from the rear end of the female housing  210  to the rear end of the lift preventing portion  227 . A finger contact portion  243  is provided substantially in a central part of the main body  241  in forward and backward directions and defines steps that climb up towards the front. The slider  240  can be slid easily using this finger contact portion  243  (see  FIG. 26 ). 
   The guiding ribs  242  are thin plates that project laterally from lower parts of the side surfaces of the main body  241  and extend from the rear of the main body  241  towards or to front. The guiding ribs  242  are inserted into clearances between the outer tube  213  of the female housing  210  and the guides  219  and are slidable in forward and backward directions to prevent a lift of the slider  240  from the female housing  210 . 
   A projection  244  projects up and out at a substantially central position of the guiding rib  242  in forward and backward directions. The projection  244  is shaped to fit into the engaging window  221  and is slidable in forward and backward directions therein. The front surface of each projection  244  is inclined to project gradually more toward the rear and a restricting surface  244 A is defined at the rear of each projection  244  for engaging the rear edge of the engaging window  221  to prevent the slider  240  from being detached backward. The projection  244  is provided at the rear end of an engaging piece  245  that is cantilevered backward along the side edge of the guiding rib  242 . The slider  240  is assembled with the female housing  210  from behind and resiliently deforms the engaging pieces  245  down and in towards the escaping recesses  222  of the female housing  210 . 
   Backlash preventing ribs  246  are provided on the side surfaces of the both guiding ribs  242  before and after the engaging pieces  245  and extend in forward and backward directions. The backlash preventing ribs  246  and the backlash preventing ribs  223  of the outer tube  213  prevent backlash between the slider  240  and the female housing  210 . 
   The main body  241  has a pressing surface  247  for sliding contact with the upper member  232  of the spring  230  to displace the spring  230  to the retracted position RP as the slider  240  is slid (see  FIG. 27 ). The pressing surface  247  is formed in a part of the main body  241  before the guiding ribs  242  and inclined up and away from the guiding ribs  242  towards the front end. Thus, the pressing surface  247  thins the main body  241  towards the front end. 
   The slider  240  slides in forward and backward directions in a widthwise central part of the female housing  210  between the standing walls  218 , and the pressing surface  247  presses a widthwise central part of the upper member  232  of the spring  230  between the legs  231 . 
   The pressing surface  247  of the slider  240  does not press the spring  230  when the slider  240  is at a rearmost position in its slidable range and the female housing  210  is in a state lockable with the male housing  250  (see  FIG. 22 ). This position of the slider  240  is called a lockable position. The pressing surface  247  of the slider  240  presses the spring  230  and displaces the spring  230  to the retracted position RP for canceling a locked state with the male housing  250  when the slider  240  is at a foremost position in its slidable range (see  FIG. 31 ). This position of the slider  240  is called an unlocking position. The projections  244  are at the rear ends of the engaging windows  221  and engage the engaging surfaces  255 A (see  FIG. 29 ) when the slider  240  is at the lockable position. However, the projections  244  are at the front ends of the engaging windows  221  (see  FIG. 30 ) when the slider  240  is at the unlocking position. The positions of the projections  244  can be seen through the engaging windows  221  to confirm visually or with an external detecting device, such as a camera, whether the slider  240  is at the lockable position or at the unlocking position. 
   A contact  248  projects forward from the front edge of the pressing surface  247  of the slider  240  and at a side of the upper member  232  of the spring  230  opposite to the female housing  210  (see  FIG. 22 ). The contact  248  extends over substantially the entire width of the main body  241  and has a dimension substantially equal to or longer than about one third of the width of the upper member  232  when the slider  240  is at the lockable position (see  FIG. 23 ). The contact  248  preferably projects more forward than the lift preventing portion  227  and is held in contact with the outer wall  251  of the device when the slider  240  is at the unlocking position (see  FIG. 31 ). 
   The spring  230  is at the inserted position IP, and the slider  240  is at the lockable position prior to locking the connectors M, F. The female housing  210  then is gripped by fingers and is oriented so that the passages  225  of the female housing  210  aligned with the engaging projections  255  of the male housing  250 . The female housing  210  then is pushed gradually into the male connector M so that the engaging projections  255  move forward in the passages  225 . As a result, the front surfaces of the engaging projections  255  contact the engaging portions  233  of the spring  230  and cause the legs  231  to displace resiliently away from each other. At this time, the contact portion  248  of the slider  240  is above the spring  230  and prevents the spring  230  from lifting in response to forces exerted on the legs  231 . 
   The projecting end surfaces of the engaging projections  255  move forward of the spring  230  when the connectors F, M reach a properly connected state. Thus, the legs  231  resiliently return so that the engaging portions  233  and the engaging surfaces  255 A face each other in forward and backward directions for locking the housings  210 ,  250  together. Inner spaces of the two housings  210 ,  250  are sealed by the seal ring  216  and rubber plugs  229  for the wires, and the male and female terminals  211 ,  254  are connected electrically when the connectors F, M reach this properly connected state. 
   The two properly connected and locked connectors F, M can be separated if necessary. More particularly, fingertips are placed on the contact  248  of the slider  240  at the lockable position to slide the slider  240  forward. Thus, the pressing surface  247  slides in contact with the upper member  232  of the spring  230  to press the spring  230  down, and the front end portion of the slider  240  is inserted into the insertion hole  228 A below the lift preventing portion  227 . The engaging portions  233  are guided outwardly along the inclinations of the lower surfaces  226 B of the communicating portions  226  as the spring  230  is pressed down by the pressing surface  247  so that the legs  231  are deformed away from each other (see  FIG. 32 ). The engaging portions  233  are at the bottom ends of the lower surfaces  226 B of the communicating portions  226  and are retracted from the passages  225  when the slider  240  reaches the unlocking position. Thus, the engaging projections  255  are freed from the locked state and can move back. At this time, the contact  248  of the slider  240  engages with the outer wall  251  of the device (see  FIG. 31 ). Fingers then are placed on the finger placing portion  217  of the female housing  210  to pull the female housing  210  back while the slider  240  is pressed against the outer wall  251  of the device so that the two connectors F, M are separated. 
   The pressing surface  247  slides in contact with the spring  230  and presses the spring  230  to the retracted position RP merely by sliding the slider  240 , thereby canceling the locked state. The spring  230  need not be operated directly by fingertips so that operability is good. The locked state can be canceled without any problem even if the outer wall  251  of the device prevents fingers from directly contacting the spring  230 , as in this embodiment. Labor and time required for an unlocking operation are saved as compared with a case where the unlocking operation is performed using a jig. 
   The female housing  210  can be pulled out in a separating direction by sliding the slider  240  so that the contact  248  engages the outer wall  251  of the device after the slider  240  and the pressing surface  247  presses the spring  230  to the retracted position RP. Thus, a connector separating operation can be performed easily, as compared with the case where the male connector M has to be pulled out while holding the female housing  210  with fingertips. 
   The slider  240  is slid to the lockable position after separating the connectors F, M. As a result, the force for pressing the spring  230  down is released and the engaging portions  233  are pressed against the lower surfaces  226 B of the communicating portion  226  by resilient restoring forces of the legs  231 . The engaging portions  233  are guided into the passages  225  by the inclinations of the lower surfaces  226 B of the communicating portions  226  so that the spring  230  is displaced from the retracted position RP to the inserted position IP. Accordingly, the spring  230  is set automatically to the inserted position IP merely by returning the slider  240  to the initial position IP. The spring  230  need not be set to the inserted position IP again for reconnecting the connectors F, M, and labor and time for the next locking operation is saved. 
   The spring  230  reaches the retracted position RP from the inserted position IP by being displaced in the direction towards the female housing  210 . In other words, the displacing direction of the spring  230  during the unlocking operation is opposite to the separating direction of the spring  230  from the female housing  210 . Thus, the spring  230  is prevented from being separated inadvertently from the female housing  210  during the unlocking operation. 
   The contact  248  of the slider  240  is above the upper member  232  of the spring  230  and prevents upward displacements of the spring  230 . Thus, displacement and backlash of the spring  230  are prevented and the connector assembly can be used under high vibration. 
   The slider  240  presses the widthwise central part of the spring  230  between the legs  231 . Thus, the force for pressing the spring  230  acts in a well-balanced manner and the unlocking operation is performed efficiently. 
   The female housing  210  has the lift preventing portion  227  to prevent lift of the front end of the slider  240  during the sliding operation. Thus, it is not necessary to press the slider  240  to ensure that the slider  240  does not lift upon pressing the spring  230  down. As a result, the unlocking operation can be performed easily. 
   A locking structure according to a further embodiment of the invention is described with reference to  FIGS. 33 to 36 . The locking structure of this embodiment differs from the previous embodiment in that the spring  230  reaches the retracted position RP from the inserted position IP by being displaced up and away from the female housing  210 . Elements that are the same as to similar to the previous embodiment are not described again, but merely are identified by the same reference numerals. 
   The locking structure of this embodiment employs a spring  230  with two resiliently displaceable leg portions  231  to lock a female connector F and the male connector M together. The spring  230  is mounted to a mounting portion  224  of a female housing  210  and is displaceable between an inserted position IP and a retracted position RP. The spring  230  is moved up and out from the inserted position IP to the retracted position RP. Engaging portions  233  of the spring  230  project into passages  225  when the spring is at the inserted position IP. However, the engaging portions  233  retract from the passages  225  and cancel a locked state when the spring  230  reaches the retracted position RP. Upper surfaces  226 A of communicating portions  226  of the female housing  210  are inclined to guide the engaging portions  233  of the spring  230  to the passages  225 . 
   A slider  290  is assembled to the female housing  210  and is slidable in forward and backward directions corresponding to a connecting direction with a male housing  250 , as in the previous embodiment. The rearmost position in a slidable range is a lockable position and the foremost position therein is an unlocking position. 
   A pressing surface  291  is formed on a main body  241  of the slider  290  for sliding contact with an upper member  232  of the spring  230  to press the upper member  232  up and out so that the spring  230  is displaced up to the retracted position RP as the slider  290  is slid. The pressing surface  291  is on a part of the main body  241  before guiding ribs  242  and inclines down towards the guiding ribs  242  and towards the front end (see  FIG. 34 ). 
   A contact  248  projects at the front of the main body  241 , as in the previous embodiment. The contact  248  is above the upper member  232  of the spring  230  when the slider  290  is at the lockable position (see  FIG. 33 ) and projects forward from a lift preventing portion  227  at the front end of the female housing  210  when the slider  290  is at the unlocking position (see  FIG. 36 ). Similar to the previous embodiment, the contact  248  contacts an outer wall  251  of the male housing  250  after the spring  230  is pressed to the retracted position. 
   An operation of locking the male and female connectors M and F in a properly connected state is performed similar to the previous embodiment. An operation of separating the properly connected connectors F, M also is performed similar to previous embodiment by sliding the slider  290 . More particularly, fingertips are placed on a finger contact  243  of the slider  290  at the lockable position to slide the slider  290  forward. The pressing surface  291  then slides in contact with the upper member  232  of the spring  230  to displace the spring  230  up. Thus, the engaging portions  233  are guided out along the inclinations of the upper surfaces  226 A of the communicating portions  226  to deform the legs  231  away from each other. The engaging portions  233  are at upper ends of the upper surfaces  226 A of the communicating portions  226  and are retracted from the passages  225  when the slider  290  reaches the unlocking position, thereby canceling the locked state. At this time, the contact  248  of the slider  290  is in contact with the outer wall  251  of the male connector M, and fingers on the finger placing portion  217  of the female housing  210  can pull the female housing  210  back to separate the connectors F, M while the slider  290  is pressed against the outer wall  251 , similar to the previous embodiment. 
   The invention is not limited to the above described and illustrated embodiments. For example, the following modifications are also embraced by the technical scope of the present invention. 
   Although the invention is applied to the locking structure of a connector assembly in the above embodiments, it can be applied to a locking structure for a pair of any members connectable with each other. 
   The locked state is canceled by sliding the slider  240  forward in the above embodiments. However, the locked state may be canceled by sliding the slider backward. At this time, the pressing surface of the slider may be inclined to press the spring to the retracted position by a backward sliding operation. 
   A locking structure according to a further embodiment of the invention is described with reference to  FIGS. 33 to 43 . The locking structure of this embodiment is applied to female and male connectors F, M. Ends of the connectors to be connected are referred to herein as the front ends and reference is made to  FIG. 33  concerning upper and lower sides. 
   The male connector M includes a male housing  350  made e.g. of synthetic resin. The male housing  350  includes wide substantially rectangular tubular front and rear receptacles  352  and  353 . The front receptacle  352  has an open front end and the rear receptacle  353  has an open rear end. Male terminals  351  penetrate a back wall  354  of the male housing  350  between the front and rear receptacles  352  and  353  and are arranged substantially side by side in the width direction. Front and rear parts of the male terminals  351  respectively project forward and backward in the front and rear receptacles  352  and  353 . 
   Engaging projections  355  project laterally from opposite left and right surfaces of the male housing  350  (see  FIG. 41 ). The engaging projections  355  are at substantially central positions of the male housing  350  in forward and backward directions, and are substantially in the vertical center of the male housing  350 . Front surfaces of the engaging projections  355  are inclined moderately out from the front towards the rear, whereas engageable surfaces  356  are defined at the rear of the engaging projections  355 . 
   The female connector F includes a female housing  310  that is made e.g. of synthetic resin and that is connectable with the male housing  350 . The female housing  310  includes a terminal accommodating portion  312  for accommodating female terminals  311  and an outer tube  313  surrounds the outer peripheral surface of the terminal accommodating portion  312 . 
   The terminal accommodating portion  312  is a wide substantially rectangular column that can fit into the front receptacle  352  of the male housing  350 . Cavities  314  are formed in the terminal accommodating portion  312  and female terminals  311  are insertable into the cavities  314  from behind. 
   The outer tube  313  extends between the rear and front ends of the terminal accommodating portion  312 , and a connecting portion  315  unitarily joins the rear end of the outer tube  313  to the rear end of the terminal accommodating portion  312 . A clearance is defined between the outer tube  313  and the terminal accommodating portion  312  and can receive the front receptacle  352  of the male housing  350 . A seal ring  316  is mounted on a back part of the outer peripheral surface of the terminal accommodating portion  312 . The seal ring  316  is squeezed between the front receptacle  352  and the terminal accommodating portion  312  to provide sealing between the housings. An opening  313 A is formed in the lower surface of the outer tube  313  to assemble a retainer  338  for retaining the female terminals  311  in the terminal accommodating portion  312 . 
   Finger placing portions  317  project laterally from rear parts of the left and right surfaces of the female housing  310 . Fingertips can be placed on the finger placing portions  317  to pull the female housing  310  for separating the connectors F, M. 
   Standing walls  318  are provided on the upper surface of the outer tube  313 . The standing walls  318  extend continuously in forward and backward directions from the rear end of the female housing  310  to a position near the front end thereof. 
   A rear recess  319  is formed in a part of the upper surface of the outer tube  313  between the standing walls  318  from an intermediate position in forward and backward directions to the rear of the female housing  310 . The depth of the rear recess  319  is increased gradually towards the rear. 
   First positioning walls  321  extend substantially circumferentially along the opposite left and right surfaces of the outer tube  313  from the front ends of the standing walls  318 . A second positioning wall  322  is provided on the outer tube  313  before the first positioning walls  321 . The second positioning wall  322  extends substantially circumferentially over the upper surface and the opposite side surfaces of the outer tube  313 . The second positioning wall  322  and the first positioning walls  321  are substantially parallel to each other and spaced in forward and backward directions (see  FIG. 39 ). 
   A front recess  323  is formed substantially in a widthwise center of the second positioning wall  322  and becomes gradually deeper from the opposite widthwise sides towards the center. A depth in a central part of the front recess  323  preferably is less than about ⅔, more preferably about half the thickness of the second positioning wall  322  in forward and backward directions. 
   A mounting portion  324  is recessed in a part of the outer tube  313  between the first positioning walls  321  and the second positioning wall  322  near the front end. 
   Passages  325  are formed in the opposite left and right walls of the outer tube  313  and can receive engaging projections  355  of the male housing  350 . Each) passage  325  extends back from the front end of the outer tube  313  and communicates with the mounting portion  324 . 
   Communicating portions  326  provide communication with the mounting portion  324  and the passages  325  and have upper surfaces  326 A and lower surfaces  326 B that incline to approach each other from the outer side towards the inner side. A vertical dimension of the communicating portions  326  at the inner sides is substantially equal to that of the engaging projections  355  (see  FIG. 41 ). 
   A downwardly-open U-shaped or W-shaped spring  340  is mounted on or to the mounting portion  324  of the female housing  310 . The spring  340  includes two legs  341  that resiliently displaceable in directions substantially facing each other and an upper member  342  connects the upper ends of the legs  341 . The legs  341  extend substantially vertically down form opposite ends of the upper member  342  in a natural unbiased state. 
   Bottom ends of the legs  341  are bent to define engaging portions  343  that converge inwardly towards each other and that project into the passages  325 . The engaging portions  343  include upper inclined sides  343 A inclined inwardly towards each other and lower inclined sides  343 B. Thus, the engaging portions  343  have a smaller extension in a vertical direction towards the inner side. Boundaries between the upper and lower inclined sides  343 A and  343 B of the engaging portions  343  are rounded. 
   The spring  340  is mounted for displacement between an inserted position IP (see  FIG. 41 ) and a retracted position RP (see  FIG. 42 ) where. The engaging portions  343  pass into the passages  325  when the spring  340  is at the inserted position IP. However, the engaging portions  343  are retracted from the passages  325  when the spring is at the retracted position RP. The engaging projections  355  move along the passages  325  during the connection process and press the engaging portions  343  in the passages  325  to deform the legs  341  outwardly. The engageable surfaces  356  pass the engaging portions  343  when the two connectors F, M reach a properly connected state. Thus, the legs  341  resiliently restore so that the engaging portions  343  engaging the engageable surfaces  356  to lock the male housing  350 . The spring  340  reaches the retracted position RP by being displaced up and out away from the female housing  310  from the inserted position IP. The engaging portions  343  are retracted from the passages  325  to cancel the locked state when the spring  340  reaches the retracted position RP. The engaging portions  343  retracted from the passages  325  are at the outer sides of the upper surfaces  326 A of the communicating portions  326 . 
   An unlocking arm  327  extends in forward and backward directions from the rear end of the female housing  310  to the front recess  323  of the second positioning wall  322  at a position between the standing walls  318 . Rotary shafts  328  connect opposite widthwise sides of the unlocking arm  327  to the standing walls  318  (see  FIGS. 38 and 39 ). Thus, the unlocking arm  327  can incline like a seesaw about rotary shafts  328 . The rotary shafts  328  are at substantially longitudinal central positions of the unlocking arm  327 . The unlocking arm  327  is substantially parallel to the upper surface of the outer tube  313  when in the natural unbiased state (see  FIG. 33 ). 
   A pressing portion  329  is defined at the front end of the unlocking arm  327  and is below the spring  340 . Thus, the pressing portion  329  can press the spring  340  up and out away from the female housing  310  when the unlocking arm  327  is inclined. A front wall  331  stands up at a front end of the unlocking arm  327 , and the pressing portion  329  can be deflected down and in by the front wall  331 . The front wall  331  enters the front recess  323  of the second positioning wall  322 . Rear surfaces of the front wall portion  331  and the second positioning wall  322  are substantially flush when the unlocking arm  327  is in a natural state. The front wall  331  has a converging shape so that the width thereof is gradually decreased toward the upper end. 
   At least one bulge  332  projects back at the upper end of the front wall  331 . A projecting distance of the bulging portion  332  is substantially equal to half the width of the spring  340  in forward and backward directions. The bulge  332  is at a side of the spring  340  opposite to the pressing portion  329  and is at a widthwise central part of the upper member  342  of the spring  340  (see  FIGS. 39 and 41 ). 
   A mount guiding surface  333  is defined at the top of the bulge  332  and inclines down toward the projecting end. Thus, the spring  340  can be mounted smoothly on the mounting portion  324 . A restraining surface  334  is defined at the lower surface of the bulge  332  and restrains the upper side  342  of the spring  340  from above. The projecting end of the restraining surface  334  is arcuate and projects slightly down to press the upper end of the spring  340 . 
   The bulge  332  is unitary with the unlocking arm  327  and a space having a dimension substantially equal to the vertical dimension of the upper member  342  of the spring  340  is defined between the suppressing surface  334  and the pressing portion  329 . Thus, a widthwise central part of the upper member  342  of the spring  340  is held between the restraining surface  334  and the pressing portion  329  with substantially no vertical clearance. 
   The spring  340  is substantially entirely held between the first positioning walls  321  and the front wall  331 , the second positioning wall  322  with substantially no clearance defined in forward and backward directions. The heights of the first and second positioning walls  321 ,  322  are set to be located before and after the spring  340  even if the spring  340  is displaced between the inserted position IP and the retracted position RP. 
   An unlocking portion  335  is defined at the rear end of the unlocking arm  327  and is wider than a part of the unlocking arm  327  before the rotary shafts  328 . A pressable slant  336  is formed on the upper surface of the unlocking portion  335  and inclines gradually up toward the rear. The lower surface of the unlocking portion  335  is inclined up towards the rear to thin the unlocking portion  335  towards the rear. The inclined lower surface of the unlocking portion  335  and the rear recess  319  of the female housing  310  ensure a sufficient downward displacement of the unlocking portion  335  while suppressing the height of the female housing  310 . 
   When the unlocking arm  327  is in the natural state, the spring  340  is held at the inserted position IP and the female housing  310  is in a state lockable with the female housing  350 . A state of the unlocking arm  327  at this time is called a lockable state (see  FIG. 33 ). When the unlocking portion  335  is pressed to incline the unlocking arm  327 , the pressing portion  329  presses the spring  340  up and the spring  340  is displaced to the retracted position RP, i.e. the locked state with the male housing  350  is canceled. This state of the unlocking arm  327  is called an unlocking state (see  FIG. 42 ). 
   The unlocking arm  327  is in the lockable state, and the spring  340  is at the inserted position IP at the start of a connecting operation. The male and female connectors M, F are connected by gripping the female housing  310  by fingers and orienting the passages  325  of the female housing  310  with respect to the engaging projections  355  of the male housing  350 . The female connector F then is pushed gradually into the male connector M. Thus, the engaging projections  355  move forward in the passages  325  and the front surfaces of the engaging projections  355  contact the engaging portions  343  of the spring  340 . The engaging projections  355  move forward and onto the engaging portions  343  to displace the legs  341  away from each other. At this time, the bulge  332  of the unlocking arm  327  is above the spring  340 . Thus, the spring  340  cannot be lifted up in response to the forces that displace the legs  341 . 
   The legs  341  resiliently return when the connectors F, M reach the properly connected state and the engaging portions  343  engage the engaging projections  355  to lock the two housings  310 ,  350  together. In the properly connected state of the two connectors F, M, the seal ring  316  and rubber plugs  337  for wires seal inner spaces of the housings  310 ,  350 , and the male and female terminals  311 ,  351  are connected electrically. 
   In the two connectors F, M properly connected in this way, the spring is held between the bulge  332  and the pressing portion  329 . Thus, vertical and circumferential displacements of the spring  340  relative to the mounting portion  324  are prevented. The spring  340  and the engaging projections  355  of the male housing  350  reliably are held engaged by preventing the displacements of the spring  340 . Thus, locking reliability can be improved. 
   In addition, the spring  340  is held closely between the restraining surface  334  and the pressing portion  329  to prevent shaking. Displacement and backlash of the spring  340  in forward and backward directions are prevented because the spring  340  is held closely between the first positioning walls  321  and the second positioning wall  322 . Since relative displacements of the spring  340  and the female housing  310  can be prevented in this way, the two connectors F, M can sustain the use in a high-vibration environment. 
   The connectors F, M are locked by placing fingertips on the pressable slant  336  of the unlocking arm  327  and pressing the unlocking portion  335  down. The unlocking arm  327  then inclines and the pressing portion  329  displaces the spring  340  up. The bulge  332  is in the widthwise center of the spring  340  prevents upward displacement of the spring  340 . Thus the spring  340  cannot displace so that one side is lifted up from the pressing portion  329 . 
   The engaging portions  343  are guided out along the inclinations of the upper surfaces  326 A of the communicating portions  326  and the legs  341  are deformed away from each other as the spring  340  is displaced up by the pressing portion  329 . The engaging portions  343  are at the upper ends of the upper surfaces  326 A of the communicating portions  326  and retracted from the passages  325  when the unlocking arm  327  reaches the unlocking state. The bulge  332  is provided above the spring  340  to ensure that the spring  340  is not separated from the pressing portion due to an excessively strong force for pulling the spring  340  up. 
   The pressing portion  329  is deflected toward the side, toward which the spring  340  is pressed, upon pressing the spring  340 . Thus, separation of the spring  340  from the pressing portion  329  during the unlocking operation can be avoided. 
   The pressing portion  329  of the unlocking arm  327  presses the widthwise central part of the spring  340 . Thus, the force for pressing the spring  340  acts on the spring  340  in a well-balanced manner and the unlocking operation can be performed efficiently. 
   When the unlocking arm  327  is operated to enable the engaging projections  355  to move backward, i.e. free them from the locked state, fingers are placed on the finger placing portions  317  of the female housing  310  and the female housing  310  is held from the substantially opposite sides and pulled backward, thereby separating the two connectors F, M. 
   Fingers pressing the unlocking portion  335  are moved away after separating the connectors F, M. Thus, the unlocking arm  327  naturally resiliently returns to the lockable state to release a force for pressing the spring  340 . The engaging portions  343  then are pressed against the upper surfaces  326 A of the communicating portions  326  by resilient restoring forces of the legs  341  and guided into the passages  325  by the inclinations of the upper surfaces  326 A. Accordingly, the spring  340  is displaced from the retracted position RP towards or to the inserted position IP and is set automatically to the inserted position IP merely by moving the fingertips away from the unlocking portion  335 . Therefore, the spring  340  need not be set to the inserted position IP again upon reconnecting the two connectors F, M and time and labor required for connecting and locking the connectors F, M can be saved. 
   The displacement of the spring  340  when the female connector F is left alone can be prevented by providing the bulge  332 . Here, if the spring  340  is displaced when the female connector F is left alone, the spring  340  has to be set to the correct position again upon connecting the female connector F with the male connector M, which requires labor and time. However, by providing the bulge  332  as in this embodiment, the displacement of the spring  340  can be prevented. Therefore, it is unnecessary to set the spring  340  again and labor and time required for the connector connecting operation can be saved. 
   The invention is not limited to the above described and illustrated embodiment. For example, the following modifications also are embraced by the technical scope of the present invention. 
   Although the present invention is applied to the locking structure of the connector device in the above embodiments, it can be applied to a locking structure for a pair of any members connectable with each other. 
   The unlocking arm  327  is made inclinable like a seesaw by the rotatable rotary shafts in the above embodiment. However, the unlocking arm may have other connections provided that it is inclinable like a seesaw. For example, a supporting leg may stand up from the upper surface of the female housing and the unlocking arm may be inclined with this supporting leg as a supporting point.