Abstract:
The invention provides a shield structure which is compatible for a plurality of helmets in spite that an auxiliary shield mounting mechanism has a comparatively simple structure, is less expensive, and provides a good appearance as a whole. According to one aspect of the invention, each of left and right auxiliary shield mounting mechanisms which are disposed on a main shield includes an inner auxiliary shield mounting portion and outer auxiliary shield mounting portion in common.

Description:
TECHNICAL FIELD 
     The present invention relates to a shied structure for a helmet or goggles, including a main shield and left and right auxiliary shield mounting mechanisms which are disposed on the main shield, in which each of the left and right auxiliary shield mounting mechanisms includes an inner auxiliary shield mounting portion. The present invention also relates to a helmet in which such a shield structure is pivotally mounted on a head protecting body. 
     The present invention also relates to a shield structure for a helmet or goggles, comprising a main shield and left and right auxiliary shield mounting mechanisms which are disposed on the main shield, in which at least one auxiliary shield mounting mechanism of the left and right auxiliary shield mounting mechanisms comprises an auxiliary shield holding member including an engaging shaft to relatively engage with one of engaging notched recess and an engaging hole of an auxiliary shield and a removal preventive portion to prevent removal of the engaging shaft from one of the engaging notched recess and the engaging hole. The present invention also relates to a helmet in which such a shield structure is pivotally mounted on a head protecting body. 
     BACKGROUND OF THE INVENTION 
     In a full-face-type helmet or the like, when a regular shield (to be referred to as a “main shield” in this DESCRIPTION) tends to fog as in the rain, a shield structure is sometimes employed in which an anti-fogging auxiliary shield referred to as an anti-fogging sheet or the like is mounted on the main shield, as disclosed in, e.g., WO 01/13750 A1 (to be referred to as “the prior patent reference” hereinafter). In the helmet shield structure (to be referred to as “the shield structure of the prior patent reference” hereinafter) disclosed in the above prior patent reference, the anti-fogging auxiliary shield is detachably mounted on the inner surface of the main shield to maintain a slight gap with respect to the main shield. In this case, substantially semicircular left and right engaging notched recesses are formed in the left and right ends, respectively, of the anti-fogging inner auxiliary shield. Left and right engaging headed shafts respectively having removal preventive heads for the anti-fogging inner auxiliary shield are disposed on the main shield to correspond to the left and right engaging notched recesses (that is, engaging slits), respectively, such that the headed shafts project on the inner surface of the main shield. Such engaging headed shafts have eccentric shaft structures so that they can adjust the tension of the anti-fogging auxiliary shield with respect to the main shield. Also, the non-eccentric shafts of the engaging headed shafts on the proximal end sides are pivotally inserted in mounting holes of the main shield from the outer surface side of the main shield. The removal preventive heads are fixed to the eccentric shafts. Hence, the tension can be adjusted by pivoting the eccentric shafts. 
     In the shield structure of the prior patent reference as described above, when mounting the anti-fogging inner auxiliary shield on the inner surface of the main shield, the left and right engaging headed shafts of the main shield are sequentially, relatively engaged with the left and right engaging notched recesses, respectively, of the anti-fogging inner auxiliary shield. This engaging may be done by sequentially, relatively introducing the left and right ends of the anti-fogging inner auxiliary shield into the eccentric shafts existing between the removable preventive heads of the left and right engaging headed shafts and the inner surface of the main shield. When removing the anti-fogging inner auxiliary shield from the main shield, the left and right engaging headed shafts may be sequentially, relatively removed from the left and right engaging notched recesses, respectively, by performing operation reverse to that for engaging. With the shield structure of the prior patent reference having the arrangement as described above, a sealed space serving as a heat-insulating layer can be formed between the outer main shield and the inner anti-fogging auxiliary shield. The heat-insulating layer can serve to decrease the temperature difference between the inner and outer sides of each of the two shields, thus anti-fogging both the main shield and anti-fogging inner auxiliary shield. 
     In the full-face-type helmet or the like, when the rider travels a bad road as in motocross, the main shield tends to become dirty with mud or the like. For this reason, a shield structure may be used in which one or a plurality of layers of mudguard auxiliary shields each called a mudguard sheet, tear-off film, disposable sheet, or the like are mounted on the outer surface of the main shield such that they can be sequentially removed, as disclosed in the homepage of SHOEI CO., LTD. (http://jp.shoei.com/products/ja/parts_list.php?parts_id=1) (to be referred to as “the prior non-patent reference” hereinafter). In the shield structure (to be referred to as “the shield structure of the prior non-patent reference” hereinafter) for the helmet on which the mudguard outer auxiliary shield is mounted in this manner, left and right engaging holes are formed in the left and right ends, respectively, of the mudguard outer auxiliary shield. Each engaging hole is formed of a substantially circular center hole and a pair of upper and lower slits extending from the center hole outward along the diameter in opposite directions. Left and right engaging headed shafts respectively having removal preventive heads for the mudguard outer auxiliary shield are disposed on the main shield to correspond to the left and right engaging holes, respectively, such that the head shafts project on the outer surface of the main shield. Such engaging headed shafts have eccentric shaft structures so that they can adjust the tension of the mudguard outer auxiliary shield with respect to the main shield. Also, set screws inserted in the mounting holes of the main shield are screwed in the non-eccentric shafts of the engaging headed shafts at the proximal end sides from the inner surface side of the main shield. This fixes the engaging headed shafts to the main shield. Hence, the set screws are loosened by pivoting the engaging headed shafts, the engaging headed shafts and set screws are entirely pivoted by an appropriate amount, and after that the engaging headed shafts are pivoted with respect to the set screws so that the set screws are screwed in and fixed to the engaging headed shafts again. Then, the tension can be adjusted. 
     In the shield structure of the prior non-patent reference, when mounting the mudguard outer auxiliary shield on the outer side of the main shield, the left and right engaging headed shafts of the main shield are sequentially, relatively engaged with the left and right engaging holes, respectively, of the mudguard outer auxiliary shield. This engagement may be performed by sequentially, relatively inserting the left and right engaging headed shafts into the left and right engaging holes, respectively, of the mudguard outer auxiliary shield which includes one or a plurality of layers. 
     In the shield structure of the prior non-patent reference described above, when removing the mudguard outer auxiliary shield (if it includes a plurality of layers, the outermost mudguard outer auxiliary shield) dirty with mud or the like from the main shield, the following operation may be performed. More specifically, first, the operator such as the helmet wearer may hold the mudguard outer auxiliary shield at a portion near its left or right end with the hand and pull it substantially forward. In this case, the left or right (in other words, either one) engaging headed shaft of the main shield is extracted relatively from the left or right engaging hole of the mudguard outer auxiliary shield. Subsequently, when the operator further pulls the mudguard outer auxiliary shield substantially forward with his hand, the right or left (in other words, the other) engaging headed shaft of the main shield is also extracted relatively from the right or left engaging hole of the mudguard auxiliary shield. As a result, the mudguard auxiliary shield can be removed from the main shield completely. 
     Assume that not only the anti-fogging inner auxiliary shield can be mounted on the main shield employed in the shield structure of the prior patent reference having the above arrangement, but also a mudguard outer auxiliary shield can be mounted on the same main shield as in the case of the main shield employed in the shield structure of the prior non-patent reference. Then, one type of main shield structure can be commonly employed in two types of helmets such as the helmet disclosed in the prior patent reference and the helmet disclosed in the prior non-patent reference. The main shield structure becomes compatible, which is preferable. In this case, left and right engaging headed shafts as those disposed on the main shield of the shield structure of the prior non-patent reference may be disposed on the main shield of the shield structure of the prior patent reference as second engaging headed shafts for the mudguard outer auxiliary shield. More specifically, the second engaging headed shafts are disposed on the main shield to project on the outer surface of the main shield. In this case, however, the main shield structure becomes complicated, and the number of components increases, leading to a high cost. In addition, to mount the anti-fogging inner auxiliary shield and mudguard outer auxiliary shield, the main shield is provided with a large number of engaging headed shafts. This degrades the appearance of the main shield (and accordingly the shield structure). 
     Assume that not only the mudguard outer auxiliary shield can be mounted on the main shield employed in the shield structure of the prior non-patent reference, but also an anti-fogging inner auxiliary shield can be mounted on the same main shield as in the case of the main shield employed in the shield structure of the prior patent reference. Then, one type of main shield structure can be commonly employed in two types of helmets such as the helmet disclosed in the prior non-patent reference and the helmet disclosed in the prior patent reference. The main shield structure becomes compatible, which is preferable. In this case, left and right engaging headed shafts as those disposed on the main shield of the shield structure of the prior patent reference may be disposed on the main shield of the shield structure of the prior non-patent reference as second engaging headed shafts for the anti-fogging inner auxiliary shield. More specifically, the second engaging headed shafts are disposed on the main shield to project on the inner surface of the main shield. In this case, however, the main shield structure also becomes complicated, and the number of components increases, leading to a high cost. In addition, to mount the mudguard outer auxiliary shield and anti-fogging inner auxiliary shield, the main shield is provided with a large number of engaging headed shafts. This degrades the appearance of the main shield (and accordingly the shield structure). 
     Furthermore, in the case of the shield structure of the prior non-patent reference, when adjusting the tension of the mudguard outer auxiliary shield with respect to the main shield, as described above, the set screws must be loosened by pivoting the engaging headed shafts, the engaging headed shafts and set screws must be entirely pivoted by an appropriate amount, and after that the engaging headed shafts must be pivoted with respect to the set screws so that the set screws are screwed in and fixed to the engaging headed shafts again. This leads to cumbersome tension adjusting operation. In particular, when traveling a bad road on a motorbike as in motocross, the mudguard outer auxiliary shield tends to become dirty with mud or the like. Therefore, if a plurality of layers of mudguard outer auxiliary shields are mounted on the outer surface of the main shield, they must be sequentially removed one by one within a comparatively short time interval. Every time an outer auxiliary shield is to be removed (in other words, within a comparatively short time interval), the tension of a new mudguard outer auxiliary shield existing under the removed mudguard outer auxiliary shield must be adjusted. In this case, adjustment of the tension of the mudguard outer auxiliary shield is further complicated. 
     SUMMARY OF THE INVENTION 
     The present invention can correct the above defects in the shield structures of the prior patent reference and prior non-patent reference as described above effectively with a comparatively simple arrangement. 
     The present invention, in its first aspect, relates to a shield structure for a helmet or goggles, including a main shield and left and right auxiliary shield mounting mechanisms which are disposed on the main shield, each of the left and right auxiliary shield mounting mechanisms including an inner auxiliary shield mounting portion, characterized in that each of the left and right auxiliary shield mounting mechanisms also comprises an outer auxiliary shield mounting portion. According to the first aspect of the present invention, each of left and right auxiliary shield mounting mechanisms comprises an inner auxiliary shield mounting portion and outer auxiliary shield mounting portion in common. Therefore, in spite that the auxiliary shield mounting mechanism has a comparatively simple structure, is less expensive, and provides a good appearance as a whole, either one of the inner auxiliary shield and the outer auxiliary shield can be selectively mounted on the main shield, or both of them can be mounted together on the main shield. Hence, the main shield structure is compatible for a plurality of types of helmets and/or a plurality of types of goggles, which is preferable. 
     According to the present invention, in the first mode of the first aspect, the inner auxiliary shield mounting portion comprises a first engaging shaft to engage with one of a first engaging notched recess and a first engaging hole of an inner auxiliary shield, and a first removal preventive portion to prevent removal of the first engaging shaft from one of the first engaging notched recess and the first engaging hole, and the outer auxiliary shield mounting portion comprises a second engaging shaft to engage with one of a second engaging notched recess and a second engaging hole of an outer auxiliary shield, and a second removal preventive portion to prevent removal of the second engaging shaft from one of the second engaging notched recess and the second engaging hole. According to the first mode of the first aspect of the present invention, the inner and outer auxiliary shields can be mounted on the main shield easily and reliably. 
     According to the present invention, in the first case of the first mode of the first aspect, the inner auxiliary shield mounting portion of at least one auxiliary shield mounting mechanism of the left and right auxiliary shield mounting mechanisms can be pivotal with respect to the main shield, and the first engaging shaft can comprise a first engaging eccentric shaft eccentric from a pivot center of the inner auxiliary shield mounting portion. According to this first case, the tension of the inner auxiliary shield with respect to the main shield can be adjusted comparatively easily and reliably. 
     According to the present invention, in the second case of the first mode of the first aspect, the outer auxiliary shield mounting portion of at least one auxiliary shield mounting mechanism of the left and right auxiliary shield mounting mechanisms can be pivotal with respect to the main shield, and the second engaging shaft can comprise a second engaging eccentric shaft eccentric from a pivot center of the outer auxiliary shield mounting portion. According to this second case, the tension of the outer auxiliary shield with respect to the main shield can be adjusted comparatively easily and reliably. 
     According to the present invention, in the first and second cases of the first mode of the first aspect, at least one auxiliary shield mounting mechanism of the left and right auxiliary shield mounting mechanisms may comprise the left auxiliary shield mounting mechanism and the right auxiliary shield mounting mechanism. 
     According to the present invention, in the second mode of the first aspect, at least one auxiliary shield mounting mechanism of the left and right auxiliary shield mounting mechanisms can comprise an auxiliary shield holding member including a second recess-projection engaging portion capable of engaging, by recess-projection engagement, with a first recess-projection engaging portion which is stationary with respect to the main shield, the auxiliary shield holding member can comprise an engaging shaft to relatively engage with one of an engaging notched recess and an engaging hole of an auxiliary shield, and a removal preventive portion to prevent removal of the engaging shaft from one of the engaging notched recess and the engaging hole, at least one auxiliary shield mounting portion of the inner auxiliary shield mounting portion and the outer auxiliary shield mounting portion can comprise the engaging shaft and the removal preventive portion, the auxiliary shield holding member can be pivotal with respect to the main shield, the engaging shaft can comprise an engaging eccentric shaft eccentric from a pivot center of the auxiliary shield holding member, and when the auxiliary shield holding member is moved forward in a direction substantially along the pivot center thereof, the second recess-projection engaging portion can disengage from the first recess-projection engaging portion. According to the second mode of the first aspect of the present invention, in at least one auxiliary shield mounting mechanism, the auxiliary shield holding member is pivoted after it is moved forward in a direction substantially along its pivot center, thus adjusting the tension of the auxiliary shield applied by the engaging eccentric shaft. Therefore, the tension of the auxiliary shield will not be adjusted unexpectedly, and can be adjusted accurately and reliably. 
     Furthermore, the present invention, in its second aspect, relates to a helmet characterized by comprising a shield structure according to the first aspect which is pivotally mounted on a head protecting body. The second aspect of the present invention can provide a helmet that can have the same effect as that achieved by the first aspect of the present invention. 
     The present invention, in its third aspect, relates to a shield structure for a helmet or goggles, comprising a main shield and left and right auxiliary shield mounting mechanisms which are disposed on the main shield, at least one auxiliary shield mounting mechanism of the left and right auxiliary shield mounting mechanisms comprising an auxiliary shield holding member including an engaging shaft to relatively engage with one of an engaging notched recess and an engaging hole of an auxiliary shield and a removal preventive portion to prevent removal of the engaging shaft from one of the engaging notched recess and the engaging hole, characterized in that the auxiliary shield holding member comprises a second recess-projection engaging portion capable of engaging, by recess-projection engagement, with a first recess-projection engaging portion which is stationary with respect to the main shield, the auxiliary shield holding member is pivotal with respect to the main shield, the engaging shaft comprises an engaging eccentric shaft eccentric from a pivot center of the auxiliary shield holding member, and when the auxiliary shield holding member is moved forward in a direction substantially along the pivot center thereof, the second recess-projection engaging portion disengages from the first recess-projection engaging portion. The third aspect of the present invention can provide the same effect as that achieved by the second mode of the first aspect of the present invention. 
     Furthermore, the present invention, in its fourth aspect, relates to a helmet characterized by comprising a shield structure according to the third aspect which is pivotally mounted on a head protecting body. The fourth aspect of the present invention can provide a helmet that can have the same effect as that achieved by the third aspect of the present invention. 
     In each of the second mode of the first aspect and the third aspect of the present invention, the first recess-projection engaging portion may substantially form an external gear shape, and the second recess-projection engaging portion may substantially form an internal gear shape. The removal preventive portion may also serve as a manipulation tab which substantially forms an external gear shape. At least one auxiliary shield mounting portion of the inner auxiliary shield mounting portion and the outer auxiliary shield mounting portion may comprise the outer auxiliary shield mounting portion. 
     In each of the first case of the second mode of the first aspect and the first mode of the third aspect of the present invention, each of the left and right auxiliary shield mounting mechanisms can comprise the auxiliary shield holding member including the second recess-projection engaging portion capable of engaging, by recess-projection engagement, with the first recess-projection engaging portion which is stationary with respect to the main shield. Also, according to the first case of the second mode of the first aspect and the first mode of the third aspect of the present invention, in each of the left auxiliary shield mounting mechanism and the right auxiliary shield mounting mechanism, the tension of the auxiliary shield applied by the engaging eccentric shaft can be adjusted. Hence, the tension can be adjusted further accurately and easily. 
     In each of the second case of the second mode of the first aspect and the second mode of the third aspect of the present invention, at least one auxiliary shield mounting mechanism of the left and right auxiliary shield mounting mechanisms can comprise a support shaft member fitted in a mounting hole of the main shield and mounted on the main shield, a first auxiliary shield holding member relatively, pivotally fitted with an inner circumferential surface of the support shaft member, and a second auxiliary shield holding member relatively, pivotally fitted with an outer circumferential surface of the support shaft member, the first auxiliary shield mounting member can comprise a first engaging shaft to relatively engage with one of a first engaging notched recess and a first engaging hole of one of an inner auxiliary shield and an outer auxiliary shield, and a first removal preventive portion to prevent removal of the first engaging shaft from one of the first engaging notched recess and the first engaging hole, and the second auxiliary shield mounting member can comprise a second engaging shaft to relatively engage with one of a second engaging notched recess and a second engaging hole of one of the outer auxiliary shield and the inner auxiliary shield, and a second removal preventive portion to prevent removal of the second engaging shaft from one of the second engaging notched recess and the second engaging hole. Also, according to the second case of the second mode of the first aspect and the second mode of the third aspect of the present invention, the inner auxiliary shield and the outer auxiliary shield can be mounted on the main shield easily and reliably with a comparatively simple arrangement. 
     In each of the second case of the second mode of the first aspect and the second mode of the third aspect of the present invention, the inner auxiliary shield may comprise an anti-fogging inner auxiliary shield, and the outer auxiliary shield may comprise a mudguard outer auxiliary shield. Also, at least one auxiliary shield mounting mechanism of the left and right auxiliary shield mounting mechanisms may comprise the left auxiliary shield mounting mechanism and the right auxiliary shield mounting mechanism. 
     The above, and other, objects, features and advantages of this invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic left side view of an entire full-face-type helmet in a state in which an anti-fogging inner auxiliary shield is mounted on the shield structure of the full-face-type helmet according to an embodiment of the present invention; 
         FIG. 2  is a schematic left side view of the entire full-face-type helmet in a state in which a mudguard outer shield is mounted on the shield structure in  FIG. 1 ; 
         FIG. 3  is a perspective view of an auxiliary shield mounting mechanism in  FIG. 1  in which an outer auxiliary shield mounting portion is in a pivot-locked state; 
         FIG. 4  is a perspective view of the auxiliary shield mounting mechanism in  FIG. 3  in which the outer auxiliary shield mounting portion is in a pivot-unlocked state; 
         FIG. 5  is a perspective view of the auxiliary shield mounting mechanism in  FIG. 3  which is seen from the direction opposite to that of  FIG. 3 ; 
         FIG. 6  is an exploded perspective view of the auxiliary shield mounting mechanism in  FIG. 3 ; 
         FIG. 7  is a perspective view of the auxiliary shield mounting mechanism in  FIG. 6  which is seen from the direction opposite to that of  FIG. 6 ; 
         FIG. 8  is a longitudinal sectional exploded perspective view of the auxiliary shield mounting mechanism in  FIG. 6 ; 
         FIG. 9  is a longitudinal sectional perspective view of the auxiliary shield mounting mechanism in  FIG. 3 ; 
         FIG. 10  is a longitudinal sectional perspective view of the auxiliary shield mounting mechanism in  FIG. 4 ; 
         FIG. 11  is a front view schematically showing the positional relationship between an inner auxiliary shield mounting portion and outer auxiliary shield mounting portion shown in  FIG. 8  and their operating states; 
         FIG. 12  is a front view schematically showing the positional relationship in the inner auxiliary shield mounting portion shown in  FIG. 8  and its operating state; and 
         FIG. 13  is a front view schematically showing the positional relationship in the outer auxiliary shield mounting portion shown in  FIG. 8  and its operating state. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment in which the present invention is applied to the shield structure of a full-face-type helmet will be described in “1. Schematic Arrangement of Helmet as a Whole”, “2. Arrangement of Shield Structure”, “3. Arrangement of Auxiliary Shield Mounting Mechanism” and “4. Operation of Auxiliary Shield Mounting Mechanism” with reference to the accompanying drawings. 
     1. Schematic Arrangement of Helmet as a Whole 
     As shown in  FIGS. 1 and 2 , a full-face-type helmet  1  includes:
     (a) a full-face-type head protecting body  2  to be worn on the head of a helmet wearer such as a motorbike rider;   (b) a shield structure  4  capable of opening/closing a window opening  3  formed in the front surface of the head protecting body  2  to oppose a portion between the forehead and chin (i.e., the central portion of the face) of the helmet wearer; and   (c) a pair of left and right chin straps (not shown) attached to the inside of the head protecting body  2 .   

     Of the head protecting body  2 , each of those portions respectively opposing the chin, forehead and the like of the helmet wearer is provided with one or a plurality of ventilators (not shown), as needed, to ventilate the head protecting body  2 . Left and right side portions of the shield structure  4  are pivotally mounted to an outer shell  6  constituting the outer wall of the head protecting body  2 , with a pair of left and right shield mounting mechanisms (in other words, main shield mounting mechanisms)  5 . The main shield mounting mechanisms  5  are not the main part of the present invention, and accordingly will not be described in detail in this DESCRIPTION. 
     As is known well, the outer shell  6  can be made of a hard material with large strength such as FRP or another synthetic resin. As shown in  FIGS. 1 and 2 , a window opening rim member  11  having a substantially U- or E-shaped section or the like is attached to a window opening  7 , formed in the outer shell  6  to form the window opening  3  of the head protecting body  2 , substantially throughout the entire circumference by adhesion with an adhesive, a double-sided adhesive tape, or the like, as is known well. As shown in  FIGS. 1 and 2 , a projection  11   a  is continuously formed on the lower end of the window opening rim member  11  along the lower end of the window opening  7  substantially horizontally. The lower end of the shield structure  4 , when it is at a full-closing position, abuts against the projection  11   a . Furthermore, a lower rim member  12  having a substantially U-shaped section or the like is attached to the lower end of the outer shell  6  substantially throughout the entire circumference by adhesion with an adhesive, a double-sided adhesive tape, or the like. As is known well, the window opening rim member  11  can be made of a highly flexible elastic material such as synthetic rubber. As is known well, the lower rim member  12  can be made of a soft material such as foamed vinyl chloride, synthetic rubber, or another soft synthetic resin. In  FIGS. 1 and 2 , reference numeral  13  denotes a finger rest integrally formed at the lower end of substantially the central portion of the shield structure  4 . The helmet wearer places his finger on the finger rest  13  when he wishes to reciprocally pivot the shield structure  4  upward and downward. 
     2. Arrangement of Shield Structure 
     As shown in  FIGS. 1 and 2 , the shield structure  4  basically includes a main shield  21  as the regular shield, and a pair of left and right auxiliary shield mounting mechanisms (in other words, assemblies of auxiliary shield mounting members)  22 . The auxiliary shield mounting mechanisms  22  are disposed on the left and right sides of the main shield  21  to oppose the left and right side portions, respectively, of the window opening  3 . As shown in  FIG. 1 , the shield structure  4  can include an anti-fogging inner auxiliary shield  23  detachably mounted on the inner surface of the main shield  21  with the pair of left and right auxiliary shield mounting mechanisms  22 . As shown in  FIG. 2 , the shield structure  4  can also include a mudguard outer auxiliary shield  24  detachably mounted on the outer surface of the main shield  21  with the pair of left and right auxiliary shield mounting mechanisms  22 . Left and right side portions of the main shield  21  of the shield structure  4  are pivotally mounted to the outer shell  6  with the pair of left and right shield mounting mechanisms  5 . The lower end of the main shield  21  abuts against the projection  11   a  of the window opening rim member  11 . The finger rest  13  is integrally formed at the lower end of substantially the central portion of the main shield  21 . The main shield  21  is provided to the helmet  1  for the purpose of windshield. If necessary, the main shield  21  may be colored to a degree that does not interfere with transparency, so that it can also serve as a blind (i.e., visor). The main shield  21  can be made of a transparent or translucent hard material such as polycarbonate or anther synthetic resin. 
     The anti-fogging inner auxiliary shield  23  shown in  FIG. 1  can be made of a transparent or translucent soft or hard material, e.g., a highly hygroscopic resin such as cellulose acetate or cellulose propionate, or another synthetic resin. The anti-fogging inner auxiliary shield  23  is detachably mounted on the main shield  21  to hold a slight gap  25  with respect to the main shield  21 . To enable this mounting, a pair of left and right tongue pieces  23   a  project from the left and right ends, respectively, of the anti-fogging inner auxiliary shield  23 , at portions slightly under substantially the central portions in substantially the vertical direction. As shown in  FIGS. 8 and 12 , the pair of left and right tongue pieces  23   a  have a pair of left and right engaging notched recesses  26 , respectively, each formed to extend backward and having the shape of a slit, a substantially semicircle, or the like. A packing projecting ridge  27  made of an elastic material such as silicone rubber is formed along the outer circumference of that region of the anti-fogging inner auxiliary shield  23  excluding the pair of left and right tongue pieces  23   a  to substantially form an loop. This allows the anti-fogging inner auxiliary shield  23  to hold the slight gap  25  with respect to the main shield  21  and to maintain the gap  25  airtight. As shown in  FIG. 6 , the main shield  21  has left and right mounting through holes  20  extending in the direction of thickness to substantially correspond to the left and right engaging notched recesses  26 . 
     The mudguard outer auxiliary shield  24  shown in  FIG. 2  can be slightly smaller than the main shield  21  and made of a transparent or translucent soft or hard material such as an olefin resin, propylpylene, or another synthetic resin. The mudguard outer auxiliary shield  24  is mounted to include one or a plurality of layers on the outer surface of the main shield  21  such that they can be sequentially removed. To enable this mounting, the mudguard outer auxiliary shield  24  has left and right engaging holes  28  near its left and right ends. As shown in  FIG. 8 , each engaging hole  28  is formed of a center hole  28   a  which is, e.g., substantially circular, and a pair of upper and lower slits  28   b  and  28   c  which extend from the center hole  28   a  outward along the diameter in opposite directions (that is, substantially upward and substantially downward). The left and right engaging holes  28  substantially correspond to the left and right mounting holes  20  formed in the main shield  21 . 
     3. Arrangement of Auxiliary Shield Mounting Mechanism 
     The left auxiliary shield mounting mechanism  22  and the right auxiliary shield mounting mechanism  22  can have substantially the same arrangement. Hence, a description will be made hereinafter on the left auxiliary shield mounting mechanism  22  with reference to  FIGS. 3 to 10 . This applies to the item of “4. Operation of Auxiliary Shield Mounting Mechanism” as well.  FIGS. 3 to 10  show the auxiliary shield mounting mechanism  22  as it is mounted on the main shield  21 . 
     As shown in  FIGS. 6 and 7 , the auxiliary shield mounting mechanism  22  includes:
     (a) a support shaft member  31  to be fitted in the mounting hole  20  of the main shield  21  from the outer surface side of the main shield  21  and mounted on the main shield  21 ,   (b) an outer auxiliary shield holding member  32  to be relatively fitted with the outer circumferential surface of the support shaft member  31  from the rear end side of the support shaft member  31 ,   (c) an inner auxiliary shield holding member  33  to be relatively fitted with the inner circumferential surface of the support shaft member  31  from the rear end side of the support shaft member  31 , and   (d) a screw member  35  such as a phillips head screw to be screwed into a hole or threaded hole  34 , formed in the inner auxiliary shield holding member  33  to extend in substantially the direction of diameter of the holding member  33 , from the front end side of the holding member  33  by self tap or thread engagement.   

     In other words, the auxiliary shield mounting mechanism  22  is formed as an assembly of four types of auxiliary shield mounting members including the support shaft member  31 , outer auxiliary shield holding member  32 , inner auxiliary shield holding member  33  and screw member  35 . 
     As shown in  FIG. 8 , a fitting recess  36  and screw insertion hole  37  are sequentially formed in the front portion of the support shaft member  31  from the front end side. A head  35   a  of the screw member  35  is to be fitted in the fitting recess  36  from the front end side of the support shaft member  31 . A shaft  35   b  of the screw member  35  is to be inserted in the screw insertion hole  37  from the front end side of the support shaft member  31 . A fitting hole  38  to fit with the inner auxiliary shield holding member  33  is formed in the support shaft member  31  to extend from the center to the rear portion, such that the fitting hole  38  is continuous to the screw insertion hole  37 . An substantially gear-shaped (more specifically, substantially external-gear-shaped) recess-projection engaging portion  41  is formed on the outer circumferential surface of the front end portion of the support shaft member  31  throughout the entire circumference. A thin-walled portion  42  serving as a main shield hold portion is formed at the rear end portion of the support shaft member  31  throughout the entire circumference. The inner circumferential surface of the main shield holding portion  42  forms a circular transcated conical surface  56 . The circular transcated conical surface  56  has a substantially circular transcated conical shape with a diameter that gradually decreases from the rear end toward the front end, so as to be continuous with the fitting hole  38  substantially smoothly. Furthermore, a step  54  is formed on the outer circumferential surface of the support shaft member  31  between an axial support portion  52  and the main shield holding portion  42 . The step  54  connects the large-diameter axial support portion  52  to the small-diameter main shield holding portion  42 . The outer circumferential surface of the main shield holding portion  42  has substantially the same diameter (in other words, forms a substantially columnar surface) in its axial direction. 
     As shown in  FIGS. 6 to 10 , the outer auxiliary shield holding member  32  has a substantially button-shaped holding member main body  43 . As the diameter of the holding member main body  43  is as small as with a diameter of about 15 mm and a thickness of about 10 mm, a substantially gear-shaped (more specifically, a substantially external-gear-shaped) removal preventive portion (in other words, a removal preventive head)  44  serving as a manipulation tab as well is formed on the outer circumferential surface of the holding member main body  43 . Thus, the operator can easily hold the holding member main body  43  with his two fingers. The holding member main body  43  has a hole  45  which is eccentric from the holding member main body  43 . Furthermore, the outer auxiliary shield holding member  32  has a cylindrical portion  46  which forms, e.g., a substantially circular cylinder. The cylindrical portion  46  extends from the rear end of the hole  45  of the holding member main body  43  further backward and is substantially concentric with the hole  45 . As shown in  FIG. 8 , the cylindrical portion  46  has a hole  47  with a diameter which is slightly smaller than that of the hole  45  of the holding member main body  43  because of the presence of a step  48 . 
     The inner circumferential surface of the hole  45  of the holding member main body  43  has a substantially gear-shaped (more specifically, substantially internal-gear-shaped) recess-projection engaging portion  51  to be adjacent to the step  48  from the front end side of the step  48 . The inner circumferential surface of the recess-projection engaging portion  51  has substantially the same shape as that of the outer circumferential surface of the recess-projection engaging portion  41  of the support shaft member  31 . When the support shaft member  31  is fitted in the holes  45  and  47  of the outer auxiliary shield holding member  32  from the front end side of the holding member  32 , the holding member main body  43  substantially opposes the recess-projection engaging portion  41  of the support shaft member  31 , and the cylindrical portion  46  substantially opposes the axial support portion (that is, the portion between the recess-projection engaging portion  41  and thin-walled holding portion  42 )  52  of the support shaft member  31 , as shown in  FIGS. 9 and 10 . As shown in  FIG. 7 , a substantially cylindrical auxiliary shield holding portion (in other words, an auxiliary shield engaging shaft or engaging eccentric shaft)  49  with a diameter slightly smaller than that of the holding member main body  43  is formed on the rear surface of the holding member main body  43  to be integral with the holding member main body  43 . Hence, a substantially ring-like step  40  is formed between the holding member main body  43  and auxiliary shield holding portion  49 . Also, the engaging eccentric shaft  49  and removal preventive head portion  44  form a headed engaging shaft. The auxiliary shield holding portion  49  has a plurality of slits  50  extending from the rear end of the holding portion  49  to near the step  40  in substantially the radial direction of the outer auxiliary shield holding member  32 . More specifically, four sets of slits  50 , each set including relatively close three slits  50 , are formed in the auxiliary shield holding portion  49  substantially equidistantly (in other words, to be shifted from each other by substantially 90°). Proximal portions  53   a  of two comparatively thin-walled springs  53  are formed among the slits  50  of each set. Distal ends  53   b  outwardly extending from the proximal portions  53   a  obliquely backward are continuously formed on the distal end sides of the two proximal portions  53   a , respectively. Hence, the large number of comparatively thin-walled springs  53  are integrally formed with the holding member main body  43  along the outer circumference. Each spring  53  forms a substantially L shape so it extends from the rear surface of the holding member main body  43  substantially backward and opens outward to extend obliquely backward. 
     The inner auxiliary shield holding member  33  includes an axially supported portion  55  to extend from the front portion toward the center of the inner holding member  33 . The axially supported portion  55  is to be fitted in the fitting hole  38  of the support shaft member  31  from the rear end side of the fitting hole  38 . The rear end portion of the outer circumferential surface of the axially supported portion  55  forms a circular transcated conical surface  57 . The circular transcated conical surface  57  has a substantially circular transcated conical shape with a diameter that gradually decreases from the rear end toward the front end, so as to substantially correspond to the inner circumferential surface of the main shield holding portion  42  of the support shaft member  31 . A second circular transcated conical surface  58  having a shape similar to a so-called step is formed at the front end of the circular transcated conical surface  57 . The axially supported portion  55  has the hole or threaded hole  34 , formed at the front end of the inner auxiliary shield holding member  33 , in the form of a blind hole to extend in the axial direction of the axially supported portion  55 . 
     As shown in  FIG. 8 , the inner auxiliary shield holding member  33  has a partition wall  61  formed integral with the axially supported portion  55  to be substantially adjacent to the rear end of the circular transcated conical surface  57 . The holding member  33  also has an engaging eccentric shaft (in other words, an engaging shaft)  62  formed integral with the partition wall  61  to be substantially adjacent to the rear end of the partition wall  61 . The engaging eccentric shaft  62  has a removal preventive portion (in other words, a removal preventive head)  63 , serving as a manipulation tab as well, formed at the rear end of the engaging eccentric shaft  62  to be integral with it. The removal preventive portion  63  constitutes the head of the engaging eccentric shaft  62 . The engaging eccentric shaft  62  and removal preventive portion  63  constitute the headed engaging shaft in the shield structure of the prior patent reference. As the removal preventive portion  63  is as small as with a maximum diameter of about 6 mm, it can form a flat polygon such as a flat hexagon, so the operator can hold it easily. The screw member  35  is inserted in the fitting recess  36  and screw insertion hole  37  from the front end side of the support shaft member  31 , and then screwed into the hole or threaded hole  34  of the inner auxiliary shield holding member  33 . As shown in  FIG. 7 , an indicator  65  projects from the partition wall  61  to indicate the tension of the inner auxiliary shield  23  applied by the holding member  33 . When the indicator  65  is directed upward or downward, it indicates that the tension of the inner auxiliary shield  23  applied by the holding member  33  is adjusted to a substantially intermediate state. 
     Assume that the axial directions of the support shaft member  31  and screw member  35  coincide with a center line (in other words, a common center line) L 1 . In this case, the axes of the hole  45  of the outer auxiliary shield holding member  32 , the recess-projection engaging portion  51 , the step  48  and the cylindrical portion  46  substantially coincide with the common center line L 1  of the auxiliary shield mounting mechanism  22 , as shown in  FIG. 8 . The axial direction of the mounting hole  20  of the main shield  21  also coincides with the common center line L 1 . The axial directions of the axially supported portion  55  of the inner auxiliary shield holding member  33 , the threaded hole  34 , the partition wall  61  and the removal preventive portion  63  serving as the manipulation tab as well also substantially coincide with the common center line L 1 . In contrast to this, the axes of the holding member main body  43  of the outer auxiliary shield holding member  32 , the tab  44  and a spring mechanism  64  which is formed of the large number of springs  53  into a substantially cylindrical shape as a whole substantially coincide with a center line (that is, a first eccentric center line) L 2 , eccentric from the common center line L 1 , for the outer auxiliary shield  24 . The axis of the engaging eccentric shaft  62  of the inner auxiliary shield holding member  33  substantially coincides with a center line (that is, a second eccentric center line) L 3 , eccentric from the common center line L 1 , for the inner auxiliary shield  23 . Note that the distance of eccentricity of the first eccentric center line L 2  from the common center line L 1  (in other words, the distance between the common center line L 1  and first eccentric center line L 2 ) can fall within a range of ½ to 4 times the distance of eccentricity of the second eccentric center line L 3  from the common center line L 1  (in other words, the distance between the common center line L 1  and second eccentric center line L 3 ), and is about 2 times in the embodiment shown in  FIG. 8 . 
     4. Operation of Auxiliary Shield Mounting Mechanism 
     An example of a procedure for mounting the auxiliary shield mounting mechanism  22  on the main shield  21  will be described in the following items (a) to (d). 
     (a) First, as shown in  FIG. 10 , the support shaft member  31  is relatively fitted in the holes  45  and  47  of the outer auxiliary shield holding member  32  from the front end side of the hole  45 . In this case, preferably, the recess-projection engaging portion  41  of the support shaft member  31  is engaged with the recess-projection engaging portion  51  of the holding member  32  so that the holding member  32  is set in a pivot-locked state (in other words, in a pivot-disabled state) with respect to the support shaft member  31 , as shown in  FIGS. 3 and 9 . The rear end face (that is, the step)  39  of the recess-projection engaging portion  41  of the support shaft member  31  can substantially abut against the step  48  of the holding member  32 . 
     (b) Subsequently, as shown in  FIG. 9 , the thin-walled portion (in other words, the main shield holding portion)  42  of the support shaft member  31  is relatively fitted in the mounting holes  20  of the main shield  21  from the rear end side of the main shield holding portion  42 . In this case, preferably, the front face of the main shield  21  is substantially abutted against the step  54  of the support shaft member  31 . 
     (c) Subsequently, as shown in  FIG. 10 , the axially supported portion  55  of the inner auxiliary shield holding member  33  is relatively fitted in the fitting hole  38  of the support shaft member  31  from the front end side of the axially supported portion  55 . In this case, preferably, the front face of the partition wall  61  of the holding member  33  is substantially abutted against the rear face of the main shield  21 . 
     (d) Subsequently, as shown in  FIG. 10 , the screw member  35  is screwed into the hole or threaded hole  34  of the inner auxiliary shield holding member  33  from the front end side of the holding member  33  by self tap or thread engagement. Because of this screwing, the main shield holding portion  42  of the support shaft member  31  rides over the circular transcated conical surfaces  58  and  57  of the holding member  33  sequentially. This increases the inner and outer diameters of the thin main shield holding portion  42 . Consequently, the outer circumferential surface of the main shield holding portion  42  is strongly pressed against the circumferential surface of the mounting hole  20  of the main shield  21 . This prevents the support shaft member  31  (and accordingly the entire auxiliary shield mounting mechanism  22 ) from idling with respect to the mounting hole  20 . Simultaneously, the main shield  21  is firmly fixed between the step  54  of the support shaft member  31  and the front face of the partition wall  61  of the inner auxiliary shield holding member  33 . This fixes the auxiliary shield mounting mechanism  22  to the main shield  21  firmly. Also, friction engagement of the main shield holding portion  42  of the support shaft member  31  with the circular transcated conical surface  57  of the holding member  33  becomes firm. As the main shield  21  relatively presses the distal end  53   b  of the spring  53  of the outer auxiliary shield holding member  32 , the outer auxiliary shield holding member  32  is biased forward. Hence, the step  48  is elastically press-mounted on a step  39  of the support shaft member  31 . 
     An example of a procedure for mounting the anti-fogging inner auxiliary shield  23  to the left and right auxiliary shield mounting mechanisms  22  mounted on the main shield  21  as shown in  FIG. 1  will be described in the following items (e) and (f). 
     (e) First, as shown in  FIGS. 11 and 12 , the engaging eccentric shafts  62  of the inner auxiliary shield holding members  33  of the left and right auxiliary shield mounting mechanisms  22  are sequentially, relatively introduced into the left and right engaging notched recesses  26  (see  FIG. 8 ), respectively, of the anti-fogging inner auxiliary shield  23 , thus relatively inserting or fitting them. In this case, the auxiliary shield  23  engages with the engaging eccentric shafts  62  at the engaging notched recesses  26 . 
     (f) Assume that the operator wishes to adjust the tension of the anti-fogging inner auxiliary shield  23  mounted on the main shield  21  as described in the above item (e). In this case, first, the operator slightly screws back the screw member  35  of the left and/or right auxiliary shield mounting mechanism  22  from the hole or threaded hole  34  of the inner auxiliary shield holding member  33 . Then, the operator holds the removal preventive head portion  63  serving as the manipulation tab as well of the holding member  33  with his two fingers and pivots the removal preventive portion  63  counterclockwise or clockwise through an appropriate angle, thus adjusting the tension of the inner auxiliary shield  23 . Then, the operator screws the screw member  35  again sufficiently into the hole or threaded hole  34 . 
     In the shield structure  4  having the arrangement as described above, a sealed space serving as a heat-insulating layer is formed between the outer main shield  21  and inner anti-fogging auxiliary shield  23 . This heat-insulating layer can serve to decrease the temperature difference between the inner and outer sides of each of the two shields  21  and  23 , thus anti-fogging the main shield  21  and anti-fogging inner auxiliary shield  23 . When removing the anti-fogging inner auxiliary shield  23  from the main shield  21 , the left and right engaging eccentric shafts  62  may be sequentially, relatively removed from the left and right engaging notched recesses  26 , respectively, by performing operation reverse to that for mounting described in the above item (e). Hence, each of the left and right auxiliary shield mounting mechanisms  22  includes an inner auxiliary shield mounting portion  66  formed of the engaging eccentric shaft (in other words, the inner shield engaging shaft)  62  and the removal preventive head portion (in other words, the inner shield removal preventive portion)  63  serving as the manipulation tab as well. 
     An example of a procedure for mounting the mudguard outer auxiliary shield  24  to the left and right auxiliary shield mounting mechanisms  22  mounted on the main shield  21  as shown in  FIG. 2  will be described in the following items (g) and (h). 
     (g) First, as shown in  FIGS. 11 and 13 , the holding member main bodies  43  (in other words, the removal preventive portions  44 ) of the outer auxiliary shield holding members  32  of the left and right auxiliary shield mounting mechanisms  22  are sequentially, relatively inserted in the left and right engaging holes  28  (see  FIG. 8 ) of one or the plurality of layers of mudguard outer auxiliary shields  24 . In this case, the auxiliary shield  24  relatively engages with the auxiliary shield holding portion  49  at its engaging hole  28 . 
     (h) When the operator wishes to adjust the tension of the mudguard outer auxiliary shield  24  mounted on the main shield  21  as described in the above item (g), first, he holds the removal preventive head portion  44  serving also as the manipulation tab of the outer auxiliary shield holding member  32  of the left and/or right auxiliary shield mounting mechanism  22  with his two fingers and pushes it in toward the main shield  21 . Accordingly, the large number of springs  53  (in other words, the substantially cylindrical spring mechanism  64 ) of the holding member  32  are strongly urged against the outer surface of the main shield  21  and are further elastically deformed. Hence, the removal preventive head portion  44  serving also as the manipulation tab moves forward (in other words, moves backward) toward the main shield  21 . The recess-projection engaging portion  51  of the holding member  32 , which relatively engages with the recess-projection engaging portion  41  of the support shaft member  31  so it is prohibited from pivoting with respect to the support shaft member  31 , also moves forward from the recess-projection engaging portion  41  toward the main shield  21 . Thus, the recess-projection engaging portion  51  is disengaged from the recess-projection engaging portion  41  and set in a state pivotal with respect to the support shaft member  31  (in other words, in a pivot-unlocked state). Subsequently, the operator pivots the removal preventive head portion  44  serving also as the manipulation tab of the holding member  32  counterclockwise or clockwise through a predetermined angle while holding it with his two fingers, thus adjusting the tension of the outer auxiliary shield  24 . After that, the operator releases his fingers from the manipulation tab  44 . Note that the recess-projection engaging portion  41  is slightly tapered from the front side toward the rear side (in other words, from the recess-projection engaging portion  41  side toward the axial support portion  52  side). Therefore, upon release of the fingers as described above, even if the recess-projection engaging portion  41  is slightly misaligned from the recess-projection engaging portion  51  in the pivoting direction, it can reliably engage with the recess-projection engaging portion  51  by the elastic restoration force of the large number of springs  53 . Even if the recess-projection engaging portion  41  and recess-projection engaging portion  51  are misaligned from each other and do not engage with each other, they can be engaged well with each other by pivoting the manipulation tab  44  slightly. 
     When the operator wishes to remove the mudguard outer shields  24  from the main shield  21  one by one, he may perform the following operation. More specifically, first, the operator pulls the outermost mudguard outer auxiliary shield  24  substantially forward by holding a portion of it which is close to the left or right end. In this case, the left or right removal preventive head portion  44  of the main shield  21  is relatively extracted from the left or right engaging hole  28  of the mudguard outer auxiliary shield  24 . Subsequently, when the operator further pulls the mudguard outer auxiliary shield  24  substantially forward with his hand, the right or left removal preventive head portion  44  of the main shield  21  is also relatively extracted from the right or left engaging hole  28  of the mudguard outer auxiliary shield  24 . Hence, where necessary, the mudguard outer auxiliary shields  24  can be completely removed one by one from the main shield  21 . 
     Hence, each of the left and right auxiliary shield mounting mechanisms  22  includes an outer auxiliary shield mounting portion  67  formed of the engaging eccentric shaft (in other words, the outer shield engaging shaft)  46  and the removal preventive head portion (in other words, the outer shield removal preventive portion)  44  serving as the manipulation tab as well. In spite that the left and right auxiliary shield mounting mechanisms  22  are separate mounting mechanisms, they include the inner auxiliary shield mounting portion  66  and outer auxiliary shield mounting portion  67  in common. According to the left and right auxiliary shield mounting mechanisms  22 , with the anti-fogging inner auxiliary shield  23  being mounted as shown in  FIG. 1  with the procedure described in the above items (e) and (f), when the mudguard outer auxiliary shield  24  is mounted as shown in  FIG. 2  with the procedure described in the above items (g) and (h), both the inner and outer auxiliary shields  23  and  24 , which are separate members, can be mounted on the single main shield  21  together. 
     Having described a specific preferred embodiment of this invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims. 
     For example, in the embodiment described above, the present invention is applied to the shield structure  4  of the full-face-type helmet  1 . However, the present invention can also be applied to the shield structure of a full-face-type helmet serving also as a jet-type helmet in which the chin cover can be raised, a jet-type helmet, a semi-jet-type helmet, or the like, and the shield structure of goggles. 
     In the above embodiment, the support shaft member  31  is formed separately of the main shield  21 . Alternatively, the support shaft member  31  can be formed integrally with the main shield  21 . 
     In the above embodiment, the spring mechanism  64  substantially having the tubular shape such as a cylindrical shape is formed integrally with the outer auxiliary shield holding member  32 . Alternatively, the spring mechanism  64  can be a spring mechanism such as a coil spring formed separately of the holding member  32 , or any other elastic biasing means. 
     In the above embodiment, the left and right auxiliary shield mounting mechanisms  22  are disposed on the main shield  21 . However, the number of auxiliary shield mounting mechanisms  22  to be disposed on the main shield  21  need not be two. Another auxiliary shield mounting mechanism  22  can also be disposed near the upper and/or lower end of the center of the main shield  21 . 
     In the above embodiment, the inner auxiliary shield  23  serves as an anti-fogging member, and the outer auxiliary shield  24  serves as a mudguard member. However, the inner and outer auxiliary shields  23  and  24  need not have these functions. For example, the outer auxiliary shield  24  may serve as an anti-fogging member, and the inner auxiliary shield  23  may serve as a mudguard member. 
     In the above embodiment, the recess-projection engaging portion  41  of the support shaft member  31  and the recess-projection engaging portion  51  of the outer auxiliary shield holding member  32  form substantially gear shapes (in other words, a substantially external-gear shape and a substantially internal-gear shape, respectively) each having a large number of projections and a large number of recesses. Alternatively, either one of the pair of recess-projection engaging portions  41  and  51  may have a large number of projections, and the remaining one of the pair of removal preventive portions  44  and  51  may have a large number of recesses corresponding to the large number of projections, respectively. It suffices as far as the pair of recess-projection engaging portions  41  and  51  can engage with each other by recess-projection engagement. 
     In the above embodiment, the inner auxiliary shield  23  is provided with the pair of left and right engaging notched recesses  26 , and the outer auxiliary shield  24  is provided with the pair of left and right engaging holes  28 . Alternatively, the inner auxiliary shield  23  can be provided with the left and/or right engaging hole  28 , and the outer auxiliary shield  24  can be provided with the left and/or right engaging notched recess  26 . 
     Furthermore, in the above embodiment, the holding member  32  serves as the outer auxiliary shield, and the holding member  33  serves as the inner auxiliary shield. Alternatively, the front and rear sides of the auxiliary shield mounting mechanism  22  may be reversed, so that the holding member  32  serves as the inner auxiliary shield and the holding member  33  serves as the outer auxiliary shield.