Patent Publication Number: US-2011074608-A1

Title: Button-key device and portable terminal apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to button-key devices and portable terminal apparatuses suitable for application to electronic apparatuses having press-to-operate button keys, such as mobile phones, personal handyphone systems (PHSs), personal digital assistants (PDAs), portable game machines, digital cameras, moving-image-capturing apparatuses, and notebook and desktop personal computers. 
     In particular, the present invention relates to a button-key device and a portable terminal apparatus in which button keys are providable in a region having, in the lateral direction of a casing, a length substantially the same as that of the casing and in which the casing has sufficient strength. 
     2. Description of the Related Art 
     Today, there are many kinds of portable apparatuses, such as mobile phones, PHSs, PDAs, portable game machines, and digital cameras. In such portable apparatuses, operations including selection of items from menus and input of characters are usually performed with button keys that are pressed by users. In general, as in exemplary related-art apparatuses disclosed in Japanese Unexamined Patent Application Publications No. 2007-141076, No. 2008-166993, and No. 2009-123632, a button-key device includes the following: key tops made of resin or the like and projecting outside of a casing through an opening (where button keys are provided) of the casing; and a rubber sheet having pushers realizing direct turning on/off of switches provided on, for example, a substrate or a flexible-printed-circuit (FPC) board housed in the casing when the key tops are pressed. 
     The rubber sheet is in contact with the inner side of the casing and the substrate or the FPC board, thereby sealing gaps produced therebetween. Thus, the rubber sheet prevents water and dust from entering the interior of the casing through the opening where the key tops are provided, that is, the rubber sheet enhances waterproofness and dustproofness. 
     SUMMARY OF THE INVENTION 
     The areas of key tops where fingers come into contact increase with the outside dimensions of the key tops. This advantageously improves the operability. On the other hand, the outside dimensions of key tops determine the size of the above-mentioned opening of the casing. Therefore, if the outside dimensions of key tops increase, the size of the opening of the casing also increases, resulting in a problematic reduction in the strength of the casing. 
     Specifically, if the outside dimensions of key tops are maximized for improved operability such that the key tops reach near ends of the casing, the size of the opening in which the key tops are to be provided is also maximized such that the opening reach near the ends of the casing. In such a case, since the casing provides its strength with the remaining portions at the ends thereof alone, it is difficult to maintain sufficient strength of the casing. 
     Moreover, to provide sufficient waterproofness and dustproofness by completely sealing the opening of the casing, the dimensions of a rubber sheet on which key tops are to be provided naturally become larger than the dimensions of the area occupied by the key tops. 
     If, however, the outside dimensions of key tops are maximized such that the key tops reach near the ends of the casing as described above, it becomes difficult to provide inside the casing a space sufficient for providing the rubber sheet whose dimensions are larger than the dimensions of the area occupied by the key tops. This problematically prevents provision of waterproofness and dustproofness using the rubber sheet. 
     The outside dimensions of key tops and the strength and waterproofness (dustproofness) of the casing trade each other off. In the related art, key tops are therefore provided at a certain distance from ends of the casing. Therefore, it is difficult to maximize the outside dimensions of the key tops such that the key tops reach near the ends of the casing. 
     Key tops, which are ones of members that determine the appearance of an apparatus, are desired to have various designs and shapes. Therefore, some of the structural problems described above may limit the designs of the key tops and the casing. 
     In light of the above, it is desirable that the present invention provides a button-key device and a portable terminal apparatus provided with a casing having sufficient strength and with sufficient waterproofness and dustproofness by using rubber sheets even if the outside dimensions of key tops are maximized such that the key tops reach near ends of the casing, and in which the allowance for the maximization of the outside dimensions of the key tops such that the key tops reach near the ends of the casing contributes to the flexible designs of the key tops and the casings. 
     To solve the above problems, according to an embodiment of the present invention, a button-key device includes the following elements: a casing having an opening provided by linearly and continuously cutting the casing from a position near one end to a position near another end, and a pair of strength-maintaining projections projecting in opposite directions from the ends, respectively, of the casing in a longitudinal direction of the opening; a first key-top unit including a projecting section projecting from the opening when the first key-top unit is fitted in the opening, an extended portion provided by extending an end of the projecting section, the end facing one of the strength-maintaining projections when the first key-top unit is fitted in the opening, and flanges projecting in opposite directions orthogonal to the longitudinal direction of the opening; a second key-top unit including a projecting section projecting from the opening when the second key-top unit is fitted in the opening, an extended portion provided by extending an end of the projecting section, the end facing the other strength-maintaining projection when the second key-top unit is fitted in the opening, and flanges projecting in opposite directions orthogonal to the longitudinal direction of the opening, the second key-top unit being provided such that a sum of lengths of the first and second key-top units in the longitudinal direction of the opening becomes substantially the same as a longitudinal length of the opening; a first water-blocking sheet provided on a bottom of the first key-top unit, the bottom being opposite the projecting section; a second water-blocking sheet provided on a bottom of the second key-top unit, the bottom being opposite the projecting section, the second water-blocking sheet having an overlapping portion that overlaps a portion of the first water-blocking sheet when the first and second key-top units are fitted in the opening; a pressing portion provided on the casing and applying a pressure to the overlapping portion by being in contact therewith; switches provided on a board housed in the casing and in correspondence with the first and second key-top units; a first pusher member provided between the first key-top unit and a corresponding one of the switches and configured to transmit to the switch a pressing force applied when the first key-top unit is pressed; and a second pusher member provided between the second key-top unit and the other corresponding switch and configured to transmit to the switch a pressing force applied when the second key-top unit is pressed. 
     In the above embodiment of the present invention, the pair of strength-maintaining projections provided on the casing project in opposite directions from the ends, respectively, of the casing in the longitudinal direction of the opening. 
     To fit the first key-top unit into the opening of the casing, the projecting section of the first key-top unit is made to project from the opening, and the first key-top unit is fitted into the opening of the casing such that the extended portion thereof is positioned above one of the strength-maintaining projections provided on the casing. 
     Likewise, to fit the second key-top unit into the opening of the casing, the projecting section of the second key-top unit is made to project from the opening, and the second key-top unit is fitted into the opening of the casing such that the extended portion thereof is positioned above the other strength-maintaining projection provided on the casing. 
     The opening of the casing is provided by linearly and continuously cutting the casing from a position near the one end to a position near the other end. The ends of the casing that define the opening are reinforced with the pair of strength-maintaining projections. 
     Therefore, the opening can be widened to extend from a position near the one end to a position near the other end of the casing while sufficient strength is provided to the ends of the casing that define the opening. Consequently, the outside dimensions of the key-top units to be provided in the opening can be increased. 
     In the above embodiment of the present invention, even if the outside dimensions of the key-top units are maximized such that the key-top units reach near the ends of the casing, sufficient strength of the casing and sufficient waterproofness and dustproofness by using the water-blocking sheets are provided. In other words, while sufficient strength of the casing and sufficient waterproofness and dustproofness by using the water-blocking sheets are provided, the outside dimensions of the key-top units can be maximized such that the key-top units reach near the ends of the casing. 
     The allowance for such maximization of the outside dimensions of the key-top units such that the key-top units reach near the ends of the casing contributes to the flexible designs of the key-top units and the casing. 
     Moreover, when the key-top units are fitted into the opening of the casing, the water-blocking sheets provided on the bottoms of the key-top units come into close contact with the edge of the opening. Thus, water and dust are prevented from entering the interior of the casing through the opening. 
     Furthermore, when the key-top units are fitted in the opening of the casing, the overlapping portion of the second water-blocking sheet provided on the bottom of the second key-top unit overlaps a portion of the first water-blocking sheet provided on the bottom of the first key-top unit. Furthermore, the pressing portion provided on the casing applies a pressure to the overlapping portion by being in contact therewith. Thus, water and dust are prevented from entering the interior of the casing through the gap produced between the key-top units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a mobile phone according to an embodiment of the present invention; 
         FIG. 2  is an exploded perspective view of the mobile phone according to the embodiment; 
         FIG. 3A  is a diagram for describing an opening provided in a front casing of the mobile phone according to the embodiment; 
         FIG. 3B  is another diagram for describing the opening provided in the front casing of the mobile phone according to the embodiment; 
         FIG. 4A  is a diagram for describing a first key-top unit provided in the opening of the front casing; 
         FIG. 4B  is another diagram for describing the first key-top unit provided in the opening of the front casing; 
         FIG. 4C  is another diagram for describing the first key-top unit provided in the opening of the front casing; 
         FIG. 4D  is another diagram for describing the first key-top unit provided in the opening of the front casing; 
         FIG. 4E  is another diagram for describing the first key-top unit provided in the opening of the front casing; 
         FIG. 5A  is a diagram for describing a second key-top unit provided in the opening of the front casing; 
         FIG. 5B  is another diagram for describing the second key-top unit provided in the opening of the front casing; 
         FIG. 5C  is another diagram for describing the second key-top unit provided in the opening of the front casing; 
         FIG. 5D  is another diagram for describing the second key-top unit provided in the opening of the front casing; 
         FIG. 5E  is another diagram for describing the second key-top unit provided in the opening of the front casing; 
         FIG. 6  is a perspective view of a pusher unit provided in the mobile phone according to the embodiment; 
         FIG. 7A  is a diagram for describing a process of fitting the first and second key-top units into the opening of the front casing; 
         FIG. 7B  is another diagram for describing the process of fitting the first and second key-top units into the opening of the front casing; 
         FIG. 7C  is another diagram for describing the process of fitting the first and second key-top units into the opening of the front casing; 
         FIG. 8  is a rear view of the front casing with the first and second key-top units fitted thereto; 
         FIG. 9A  is a diagram for describing a state where the first and second key-top units fitted in the opening of the front casing overlap each other for sealing therebetween; 
         FIG. 9B  is another diagram for describing the state where the first and second key-top units fitted in the opening of the front casing overlap each other for sealing therebetween; 
         FIG. 10  is a diagram for describing a state where a portion where the first and second key-top units overlap each other is pressed by a casing and how the pusher unit guides light; 
         FIG. 11  is a front view of a mobile phone according to a variation of the mobile phone according to the embodiment of the present invention; 
         FIG. 12A  is a diagram for describing an intermediate key-top unit provided in the mobile phone according to the variation together with the first and second key-top units; 
         FIG. 12B  is another diagram for describing the intermediate key-top unit provided in the mobile phone according to the variation together with the first and second key-top units; 
         FIG. 12C  is another diagram for describing the intermediate key-top unit provided in the mobile phone according to the variation together with the first and second key-top units; 
         FIG. 12D  is another diagram for describing the intermediate key-top unit provided in the mobile phone according to the variation together with the first and second key-top units; 
         FIG. 12E  is another diagram for describing the intermediate key-top unit provided in the mobile phone according to the variation together with the first and second key-top units; 
         FIG. 13  is an exploded perspective view of the mobile phone according to the variation; 
         FIG. 14  is a perspective view of a pusher unit provided in the mobile phone according to the variation; 
         FIG. 15A  is a diagram for describing a process of fitting the first, second, and intermediate key-top units to the front casing of the mobile phone according to the variation; 
         FIG. 15B  is another diagram for describing the process of fitting the first, second, and intermediate key-top units to the front casing of the mobile phone according to the variation; 
         FIG. 15C  is another diagram for describing the process of fitting the first, second, and intermediate key-top units to the front casing of the mobile phone according to the variation; 
         FIG. 15D  is another diagram for describing the process of fitting the first, second, and intermediate key-top units to the front casing of the mobile phone according to the variation; 
         FIG. 16  is a rear view of the front casing of the mobile phone according to the variation with the first, second, and intermediate key-top units fitted thereto; 
         FIG. 17A  is a diagram for describing a state where the first, second, and intermediate key-top units fitted in the opening of the front casing overlap each other for sealing therebetween; 
         FIG. 17B  is another diagram for describing the state where the first, second, and intermediate key-top units fitted in the opening of the front casing overlap each other for sealing therebetween; 
         FIG. 18A  is a diagram for describing a portion where water-blocking sheets provided on the first and second key-top units, respectively, overlap each other; 
         FIG. 18B  is another diagram for describing the portion where the water-blocking sheets provided on the first and second key-top units, respectively, overlap each other; 
         FIG. 18C  is another diagram for describing the portion where the water-blocking sheets provided on the first and second key-top units, respectively, overlap each other; 
         FIG. 18D  is another diagram for describing the portion where the water-blocking sheets provided on the first and second key-top units, respectively, overlap each other; 
         FIG. 18E  is another diagram for describing the portion where the water-blocking sheets provided on the first and second key-top units, respectively, overlap each other; 
         FIG. 18F  is another diagram for describing the portion where the water-blocking sheets provided on the first and second key-top units, respectively, overlap each other; 
         FIG. 19  is a sectional view of the mobile phone according to the embodiment taken in the longitudinal direction thereof, showing relevant parts including the second key-top unit; and 
         FIG. 20  is a sectional view of the mobile phone according to the embodiment taken in the lateral direction thereof, showing relevant parts including the second key-top unit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is applicable to mobile phones. 
     Configuration of Mobile Phone 
       FIG. 1  is a front view of a mobile phone according to an embodiment of the present invention.  FIG. 2  is an exploded perspective view of the mobile phone according to the embodiment. As can be seen from  FIGS. 1 and 2 , the mobile phone according to the embodiment has a casing formed by a front casing  1 , a rear casing  2 , and a back cover  3 . A first key-top unit  4  and a second key-top unit  5  are fitted to the front casing  1 . 
     A board  6  is housed in the casing. The board  6  is provided with a sheet key  9 . The sheet key  9  is provided with two light-emitting diodes (LEDs)  7  that illuminate the key-top units  4  and  5 , respectively, and dome switches  8  provided in correspondence with the key-top units  4  and  5 . The board  6  is fitted to a frame  10  forming an internal structural body of the mobile phone, thereby being housed in the casing. The frame  10  is provided with a display unit  11  and a pusher-unit mount  12 . A pusher unit that functions to push the dome switches  8  on the sheet key  9  is mounted on the pusher-unit mount  12 . The front casing  1  is provided with a touch panel module  13  positioned over the display unit  11  provided on the frame  10 . 
       FIG. 3A  is a front view of the front casing  1  without the first and second key-top units  4  and  5 .  FIG. 3B  is a sectional view of the front casing  1 , shown in  FIG. 3A , taken along the line IIIB-IIIB. As can be seen from  FIGS. 3A and 3B , the front casing  1  has an opening  14 . The opening  14  is provided by linearly and continuously cutting the front casing  1  from a position proximate to an end  1   a  to a position proximate to another end  1   b  of the front casing  1 . 
     The front casing  1  also has strength-maintaining projections  15   a  and  15   b  projecting in opposite directions from the ends  1   a  and  1   b , respectively, in the longitudinal direction of the opening  14 . The strength-maintaining projections  15   a  and  15   b  are each shaped such that a gap corresponding to a depth to which the key-top unit  4  or  5  is to be pressed is provided with respect to an extended portion, which will be described below, of the key-top unit  4  or  5 . 
       FIGS. 4A to 4E  are a front view, a top view, a bottom view, a right-side view, and a left-side view, respectively, of the first key-top unit  4  to be fitted into the opening  14 . As can be seen from  FIGS. 4A to 4E , the first key-top unit  4  includes a projecting section  20  that is to project from the opening  14  when the first key-top unit  4  is fitted into the opening  14 . 
     The first key-top unit  4  also includes an extended portion  21  and flanges  22 . Ends of the projecting section  20  in the longitudinal direction of the opening  14  are referred to as an end  20   a  and an end  20   b . The end  20   a  is extended to form the extended portion  21 . When the first key-top unit  4  is fitted into the opening  14 , the end  20   a  faces the strength-maintaining projection  15   a  with the aforementioned gap interposed therebetween, the gap corresponding to the depth to which the first key-top unit  4  is to be pressed. The flanges  22  project in opposite directions that are orthogonal to the longitudinal direction of the opening  14  so that the first key-top unit  4  does not come off from the opening  14  when the projecting section  20  is made to project from the opening  14 . 
     The first key-top unit  4  is provided on the bottom thereof, i.e., a surface thereof opposite the projecting section  20 , with a first water-blocking sheet  25  bonded thereto. The first water-blocking sheet  25  has a larger area than the bottom surface of the first key-top unit  4 , and is made of a waterproof rubber material or the like processed into a substantially rectangular shape with a pair of securing tabs  26  and  27  at which the first water-blocking sheet  25  is secured to the front casing  1  together with the first key-top unit  4 . Furthermore, as shown in  FIG. 4C , the first water-blocking sheet  25  has shading treatment done with, for example, black ink along the periphery thereof. With the shading treatment, light from a corresponding one of the LEDs  7  is efficiently guided toward the first key-top unit  4 . 
       FIGS. 5A to 5E  are a front view, a top view, a bottom view, a right-side view, and a left-side view, respectively, of the second key-top unit  5  to be fitted into the opening  14 . As can be seen from  FIGS. 5A to 5E , the second key-top unit  5  includes a projecting section  30  that is to project from the opening  14  when the second key-top unit  5  is fitted into the opening  14 . 
     The second key-top unit  5  also includes an extended portion  31  and flanges  32 . Ends of the projecting section  30  in the longitudinal direction of the opening  14  are referred to as an end  30   a  and an end  30   b . The end  30   b  is extended to form the extended portion  31 . When the second key-top unit  5  is fitted into the opening  14 , the end  30   b  faces the strength-maintaining projection  15   b  with the aforementioned gap interposed therebetween, the gap corresponding to the depth to which the second key-top unit  5  is to be pressed. The flanges  32  project in opposite directions that are orthogonal to the longitudinal direction of the opening  14  so that the second key-top unit  5  does not come off from the opening  14  when the projecting section  30  is made to project from the opening  14 . 
     The second key-top unit  5  is provided on the bottom thereof, i.e., a surface thereof opposite the projecting section  30 , with a second water-blocking sheet  35  bonded thereto. The second water-blocking sheet  35  has a larger area than the bottom surface of the second key-top unit  5 , and is made of a waterproof rubber material or the like processed into a substantially rectangular shape with a pair of securing tabs  36  and  37  at which the second water-blocking sheet  35  is secured to the front casing  1  together with the second key-top unit  5 . 
     Furthermore, the second water-blocking sheet  35  has an overlapping portion  38  that overlaps a portion of the first water-blocking sheet  25  when the second key-top unit  5  is fitted into the opening  14  together with the first key-top unit  4 . 
     Furthermore, as shown in  FIG. 5C , the second water-blocking sheet  35  has shading treatment done with, for example, black ink along the periphery thereof. With the shading treatment, light from a corresponding one of the LEDs  7  is efficiently guided toward the second key-top unit  5 . 
     The first and second key-top units  4  and  5  have substantially the same length and are shaped such that, in the longitudinal direction of the opening  14 , the sum of the lengths of the key-top units  4  and  5  becomes substantially the same as the length of the opening  14  in a state where the key-top units  4  and  5  are fitted in the opening  14 . 
     In this embodiment, the key-top units  4  and  5  have substantially the same length. Alternatively, as long as the sum of the lengths of the key-top units  4  and  5  is substantially the same as the length of the opening  14  in the longitudinal direction of the opening  14 , the first key-top unit  4  may be longer than the second key-top unit  5 , or conversely the second key-top unit  5  may be longer than the first key-top unit  4 . 
     The pusher unit to be mounted on the pusher-unit mount  12  of the frame  10  shown in  FIG. 2  includes, referring now to  FIG. 6 , a connecting bar  40  having a substantially stick-like shape, a first pusher member  41 , and a second pusher member  42 . The first pusher member  41  has a first projection  46  and is to be positioned between the first key-top unit  4  and the dome switch  8  corresponding thereto. When the first key-top unit  4  is pressed, the pressing force is transmitted to the dome switch  8  through the first projection  46 . The second pusher member  42  has a second projection  47  and is to be positioned between the second key-top unit  5  and the dome switch  8  corresponding thereto. When the second key-top unit  5  is pressed, the pressing force is transmitted to the dome switch  8  through the second projection  47 . 
     The pusher unit also includes a first supporting portion  43 , a second supporting portion  44 , and a pair of locking hooks  45 . The first supporting portion  43  connects the first pusher member  41  to the connecting bar  40  and bends in response to the pressing of the first key-top unit  4 . The second supporting portion  44  connects the second pusher member  42  to the connecting bar  40  and bends in response to the pressing of the second key-top unit  5 . The locking hooks  45  are provided on the connecting bar  40  and secure the pusher unit to the frame  10 . 
     The pusher unit is formed such that the connecting bar  40 , the first pusher member  41 , the first supporting portion  43 , the second pusher member  42 , and the second supporting portion  44  are integrally molded from, for example, a transparent or translucent light-conducting material, such as polycarbonate resin, that bends when a force is applied thereto and restores the original shape when the applied force is removed. 
       FIGS. 7A to 7C  show a process of fitting the key-top units  4  and  5  to the front casing  1 .  FIGS. 7A to 7C  are sectional views of the front casing  1  taken along the opening  14 . 
     To fit the first key-top unit  4  to the front casing  1 , referring to  FIG. 7A , the projecting section  20  of the first key-top unit  4  is inserted into the opening  14  from the back of the front casing  1 . Thus, the projecting section  20  of the first key-top unit  4  projects from the opening  14 , with the flanges  22  of the first key-top unit  4  being caught at the opening  14 . In this state, the first key-top unit  4  is slid toward the strength-maintaining projection  15   a  of the front casing  1  so that the extended portion  21  of the first key-top unit  4  is positioned above the strength-maintaining projection  15   a.    
     Subsequently, referring to  FIG. 7B , the extended portion  31  of the second key-top unit  5  is hooked over the strength-maintaining projection  15   b  from the back of the front casing  1 . In this state, the second key-top unit  5  is pressed from the bottom thereof toward the opening  14 . Thus, the projecting section  30  of the second key-top unit  5  projects from the opening  14 , with the flanges  32  of the second key-top unit  5  being caught at the opening  14 . 
     When the first key-top unit  4  is slid toward the strength-maintaining projection  15   a  of the front casing  1  such that the extended portion  21  is positioned above the strength-maintaining projection  15   a  and subsequently the extended portion  31  of the second key-top unit  5  is hooked over the strength-maintaining projection  15   b  of the front casing  1  and then the second key-top unit  5  is pressed from the bottom thereof toward the opening  14 , referring now to  FIG. 8 , the pair of securing tabs  26  and  27  of the first water-blocking sheet  25  and the pair of securing tabs  36  and  37  of the second water-blocking sheet  35  engage with projections  50 , respectively, provided on the front casing  1 . Thus, the key-top units  4  and  5  are secured to the front casing  1  at the opening  14 . 
     The opening  14  of the front casing  1  is provided by linearly and continuously cutting the front casing  1  from a position near the end  1   a  to a position near the end  1   b  of the front casing  1 . Furthermore, the ends  1   a  and  1   b  of the front casing  1  that define the opening  14  are reinforced with the pair of strength-maintaining projections  15   a  and  15   b.    
     Therefore, the opening  14  can be widened to extend from a position near the end  1   a  to a position near the end  1   b  of the front casing  1  while sufficient strength is provided to the ends  1   a  and  1   b  of the front casing  1  that define the opening  14 . Consequently, the outside dimensions of the key-top units  4  and  5  to be provided in the opening  14  can be increased. 
     The allowance for such maximization of the outside dimensions of the key-top units  4  and  5  such that the key-top units  4  and  5  reach near the ends  1   a  and  1   b  of the front casing  1  contributes to the flexible designs of the key-top units  4  and  5  and the casing. 
     Moreover, when the key-top units  4  and  5  are fitted into the opening  14  of the front casing  1 , the water-blocking sheets  25  and  35  provided on the bottoms of the key-top units  4  and  5  come into close contact with the edge of the opening  14 . Thus, water and dust are prevented from entering the interior of the casing through the opening  14 . 
       FIG. 9A  is a side view of the key-top units  4  and  5  fitted in the opening  14  of the front casing  1 .  FIG. 9B  is a bottom view of the key-top units  4  and  5  fitted in the opening  14  of the front casing  1 . As can be seen from  FIGS. 9A and 9B , when the key-top units  4  and  5  are fitted in the opening  14  of the front casing  1 , the overlapping portion  38  of the second water-blocking sheet  35  provided on the second key-top unit  5  overlaps a portion of the first water-blocking sheet  25  provided on the first key-top unit  4 . A pressing portion  70 , shown in  FIG. 2  and described below, provided on the frame  10  comes into contact with the overlapping portion  38 . With the pressing portion  70  pressing the overlapping portion  38 , water and dust are prevented from entering the interior of the casing through a gap between the key-top units  4  and  5 . 
     A process of mounting the pusher unit onto the frame  10  will now be described. Referring to  FIG. 2 , the frame  10  has the pusher-unit mount  12  including a groove  60  that receives the connecting bar  40  of the pusher unit, described above referring to  FIG. 6 , a concave-shaped first-pusher-member-receiving portion  61  that receives the first pusher member  41 , and a concave-shaped second-pusher-member-receiving portion  62  that receives the second pusher member  42 . 
     The first-pusher-member-receiving portion  61  has a hole  63  into which the first projection  46  of the first pusher member  41  is to be inserted. The second-pusher-member-receiving portion  62  has a hole  64  into which the second projection  47  of the second pusher member  42  is to be inserted. 
     The groove  60  is provided with locking holes (not shown) to which the pair of locking hooks  45  of the pusher unit shown in  FIG. 6  are to be hooked, respectively. 
     To mount the pusher unit onto the pusher-unit mount  12 , the connecting bar  40  of the pusher unit is fitted into the groove  60 , the first projection  46  of the first pusher member  41  is then inserted into the hole  63  of the first-pusher-member-receiving portion  61 , and the first pusher member  41  is then fitted into the first-pusher-member-receiving portion  61 . Meanwhile, the second projection  47  of the second pusher member  42  is inserted into the hole  64  of the second-pusher-member-receiving portion  62 , and the second pusher member  42  is then fitted into the second-pusher-member-receiving portion  62 . 
     Thus, the pair of locking hooks  45  shown in  FIG. 6  are hooked into the locking holes provided in the groove  60 . Consequently, the pusher unit is secured to the pusher-unit mount  12  of the frame  10 . 
     To assemble the mobile phone, with the key-top units  4  and  5  fitted to the front casing  1  and the pusher unit mounted on the pusher-unit mount  12  of the frame  10  as described above, the frame  10  and the board  6  are housed together in the casing formed by the front casing  1  and the rear casing  2 , shown in  FIG. 2 , and the back cover  3  is then put to the rear casing  2 . Thus, the mobile phone is assembled. 
     As shown in  FIG. 2 , a portion of the frame  10  between the first-pusher-member-receiving portion  61  and the second-pusher-member-receiving portion  62  forms the pressing portion  70 , which has a convex shape. When the frame  10  is placed inside the casing formed by the front casing  1  and the rear casing  2 , the pressing portion  70  comes into contact with the overlapping portion  38  where the key-top units  4  and  5  fitted to the front casing  1  overlap each other, thereby pressing the overlapping portion  38  with a specific pressure. 
     The overlapping portion  38  will now be described in detail with reference to  FIGS. 18A to 18F .  FIG. 18A  shows the second water-blocking sheet  35  provided on the second key-top unit  5  seen from the bottom of the second key-top unit  5 .  FIG. 18B  shows the second water-blocking sheet  35  seen from the right side of the second key-top unit  5 .  FIG. 18C  shows the second water-blocking sheet  35  seen from the left side of the second key-top unit  5 .  FIG. 18D  shows the second water-blocking sheet  35  seen from the side of the securing tabs  36  and  37 .  FIG. 18E  shows the first water-blocking sheet  25  provided on the first key-top unit  4  seen from the side of the securing tabs  26  and  27 .  FIG. 18F  shows the first and second water-blocking sheets  25  and  35  seen from the side of the securing tabs  26 ,  27 ,  36 , and  37  with portions thereof overlapping each other. 
     As can be seen from  FIGS. 18B to 18D , the second water-blocking sheet  35  has H-shaped longitudinal and lateral sections. That is, the second water-blocking sheet  35  has peripheral edges thereof projecting upward and downward. Likewise, referring now to  FIG. 18E , the first water-blocking sheet  25  has H-shaped longitudinal and lateral sections. 
     Referring to  FIG. 18D , the overlapping portion  38  of the second water-blocking sheet  35 , i.e., the portion of the second water-blocking sheet  35  that overlaps the first water-blocking sheet  25 , forms a tongue  115  projecting from a lower position of the second water-blocking sheet  35  in the direction opposite the direction in which the second water-blocking sheet  35  extends. Referring now to  FIG. 18E , the portion of the first water-blocking sheet  25  that overlaps the second water-blocking sheet  35  forms a tongue  116  projecting from an upper position of the first water-blocking sheet  25  in the direction opposite the direction in which the first water-blocking sheet  25  extends. 
     When the first and second water-blocking sheets  25  and  35  overlap each other, referring now to  FIG. 18F , the tongue  116  projecting from the upper position of the first water-blocking sheet  25  in the above-described direction is placed on the tongue  115  projecting from the lower position of the second water-blocking sheet  35  in the above-described direction. Thus, the water-blocking sheets  25  and  35  overlap each other at the overlapping portion  38 . 
       FIG. 10  is an enlarged sectional view of the overlapping portion  38  and relevant parts. As can be seen from  FIG. 10 , when the frame  10  is placed inside the casing formed by the front casing  1  and the rear casing  2 , the pressing portion  70 , having a convex shape, provided on the frame  10  comes into contact with the overlapping portion  38  where the key-top units  4  and  5  fitted to the front casing  1  overlap each other, thereby pressing the overlapping portion  38 . 
       FIG. 19  is a sectional view of the mobile phone taken in the longitudinal direction thereof, showing relevant parts including the second key-top unit  5 .  FIG. 20  is a sectional view of the mobile phone taken in the lateral direction thereof, showing relevant parts including the second key-top unit  5 . As can be seen from  FIGS. 19 and 20 , since the second water-blocking sheet  35  provided on the second key-top unit  5  has the H-shaped longitudinal and lateral sections with the peripheral edges thereof projecting upward and downward as described above, the edges projecting upward are in close contact with the front casing  1 , and the edges projecting downward are in close contact with the frame  10 . Likewise, the peripheral edges of the first water-blocking sheet  25  provided on the first key-top unit  4  are in close contact with the front casing  1  and the frame  10 . 
     In the state where the key-top units  4  and  5  are fitted in the opening  14  next to each other, a small gap  71  shown in  FIG. 10  is produced between the key-top units  4  and  5 , and other small gaps shown in  FIGS. 19 and 20  are produced between the edge of the opening  14  and the peripheries of the key-top units  4  and  5 . These gaps may allow water and dust to enter the interior of the casing. For easier recognition, the gap  71  in  FIG. 10  and the gaps in  FIGS. 19 and 20  are shown larger than actual scales. 
     In the mobile phone according to the embodiment, however, since the H-shaped edges of the water-blocking sheets  25  and  35  provided on the key-top units  4  and  5  are in close contact with the front casing  1  and the frame  10  in such a manner as to surround the key-top units  4  and  5 , water and dust are prevented from entering the interior of the casing through the gaps between the edge of the opening  14  and the key-top units  4  and  5 . 
     Furthermore, since the overlapping portion  38  where the tongue  116  of the first water-blocking sheet  25  and the tongue  115  of the second water-blocking sheet  35  overlap each other is provided at the position corresponding to the gap  71  between the key-top units  4  and  5 , water and dust are prevented from entering the interior of the casing through the gap  71 . Moreover, the convex-shaped pressing portion  70  on the frame  10  that is in contact with and pressed against the overlapping portion  38  enhances the prevention of entrance of water and dust into the interior of the casing through the gap  71 . 
     In addition, elastic sealing members made of a rubber material or the like may be provided in the gaps produced between the extended portions  21  and  31  of the key-top units  4  and  5  and the respective strength-maintaining projections  15   a  and  15   b  of the front casing  1  when the key-top units  4  and  5  are fitted in the opening  14 . This further enhances the prevention of entrance of water and dust into the interior of the casing through the gaps. 
     The convex-shaped pressing portion  70  provided between the first- and second-pusher-member-receiving portions  61  and  62  of the frame  10  according to the embodiment may alternatively be provided on the board  6  or the rear casing  2 , for example. That is, the pressing portion  70  may be provided at any position on the casing, except the front casing  1 , as long as the pressing portion  70  has a convex shape that presses the overlapping portion  38 . 
     In the mobile phone described above, the key-top units  4  and  5  function as, for example, a general-menu-displaying key and an on-hook/off-hook phone key, respectively. When the second key-top unit  5  is pressed, referring to  FIG. 10 , the pressing force applied to the second key-top unit  5  is transmitted to the second pusher member  42 ; the second supporting portion  44  connecting the second pusher member  42  to the connecting bar  40  bends; and the second projection  47  presses the corresponding dome switch  8  on the board  6 . Thus, an on-hook or off-hook phone operation is performed. 
     Likewise, when the first key-top unit  4  is pressed, the pressing force applied to the first key-top unit  4  is transmitted to the first pusher member  41 ; the first supporting portion  43  connecting the first pusher member  41  to the connecting bar  40  bends; and the first projection  46  presses the corresponding dome switch  8  on the board  6 . Thus, the general menu of the mobile phone is made to appear or disappear. 
     Key tops are often decorated with coating, printing, and the like for improved appearances thereof. In addition to such decoration, key tops are often provided with icons, such as characters, numbers, and symbols, representing individual operations. Furthermore, to help operations in dark environment, key tops are in general made of a material capable of guiding light from LEDs provided on a board or the like housed in a casing. Thus, for each key top, a contrast of illumination is produced between the icon and the other part, providing sufficient viewability of the icon. 
     In a case, however, where there is a substantial distance in the thickness direction of a casing between a board, having LEDs and housed in the casing, and key tops, it is difficult to form pusher structures that realize direct turning on/off of dome switches provided on the board by using the key tops alone. Moreover, the illumination by the LEDs mounted on the board for illumination of the key tops may become unstable. 
     To solve such problems, the mobile phone according to the embodiment includes the pusher unit having a pusher function and a light-guiding function is fitted to the frame  10 , as described above referring to  FIG. 6 . 
     Specifically, the first and second pusher members  41  and  42  of the pusher unit shown in  FIG. 6  have bearing surfaces  41   a  and  42   a  that receive the key-top units  4  and  5 , respectively. Thus, the pusher unit provides a pusher structure that transmits pressing forces applied to the key-top units  4  and  5  to the dome switches  8  through the first and second projections  46  and  47 , respectively. 
     In the pusher structure, the first and second supporting portions  43  and  44  are connected to the connecting bar  40  in a so-called cantilever manner. This allows the first and second supporting portions  43  and  44  to bend when the key-top units  4  and  5  are pressed, respectively. Thus, the pressing forces are transmitted to the dome switches  8 , realizing on/off operations of the dome switches  8 . 
     In addition, the pusher unit is made of a transparent or translucent material suitable for guiding light, and has bevels  75 , shown in  FIG. 10 , angled at specific degrees so as to guide and diffuse light from the LEDs  7  toward the key-top units  4  and  5 . Therefore, the light from the LEDs  7  is evenly and stably guided and diffused toward the key-top units  4  and  5 . 
     With the pusher unit having a pusher function and a light-guiding function and fitted to the frame  10 , the mobile phone according to the embodiment provides a pusher structure that realizes direct turning on/off of the dome switches  8  on the board  6  even if there is a substantial distance in the thickness direction of the casing between the board  6 , having the LEDs  7  and housed in the casing, and the key-top units  4  and  5 . Furthermore, the mobile phone according to the embodiment realizes a light-guiding function of evenly and stably guiding and diffusing the light from the LEDs  7  on the board  6  toward the key-top units  4  and  5 . 
     Advantageous Effects of the Embodiment 
     As is obvious from the above description, the mobile phone according to the embodiment provides the following advantageous effects. 
     1. With a resin connecting structure including the extended portions  21  and  31  of the first and second key-top units  4  and  5  and the pair of strength-maintaining projections  15   a  and  15   b  at the ends  1   a  and  1   b  of the front casing  1  defining the opening  14 , a sufficient structural base and strength of the casing are provided. 
     2. If an integral structure in which the first and second key-top units  4  and  5  are bonded to a single water-blocking sheet is employed, it is difficult to fit the key-top units  4  and  5  into the front casing  1  from the back of the front casing  1  because the key-top units  4  and  5  interfere with the strength-maintaining projections  15   a  and  15   b , respectively. In the mobile phone according to the embodiment, however, the key-top units  4  and  5  are separate components, and it is therefore easy to fit the key-top units  4  and  5  into the opening  14  of the front casing  1  having the strength-maintaining projections  15   a  and  15   b.    
     3. The substantial distance in the thickness direction of the casing between the key-top units  4  and  5  and the dome switches  8  on the board  6  makes it difficult to provide sufficient waterproofness and dustproofness by using the key-top units  4  and  5  alone. In the mobile phone according to the embodiment, however, when the mobile phone is assembled by fitting the key-top units  4  and  5  into the opening  14 , the edges (the upward and downward projections forming the H-shaped sections) of the water-blocking sheets  25  and  35  provided on the respective key-top units  4  and  5  come into close contact with the front casing  1  and the frame  10 . This forms a closed sealing structure. Thus, water and dust are prevented from entering the interior of the casing through the gap between the edge of the opening  14  and the key-top units  4  and  5 , whereby sufficient waterproofness and dustproofness are provided. 
     4. With the overlapping portion  38  formed by the tongue  115  of the second water-blocking sheet  35  that overlaps the tongue  116  of the first water-blocking sheet  25  at the position corresponding to the gap  71  between the key-top units  4  and  5 , water and dust are prevented from entering the interior of the casing through the gap  71  between the key-top units  4  and  5 . Moreover, the convex-shaped pressing portion  70  of the frame  10  that is in contact with and presses the overlapping portion  38  enhances the prevention of entrance of water and dust into the interior of the casing through the gap  71 . 
     5. With the substantial distance in the thickness direction of the casing between the key-top units  4  and  5  and the dome switches  8  on the board  6 , it is difficult to realize direct on/off operations of the dome switches  8  on the board  6  and to guide the light from the LEDs  7  by using the key-top units  4  and  5  alone. In the mobile phone according to the embodiment, however, a pusher structure and a light-guiding structure are realized with a single component, i.e., the pusher unit having a pusher function and a light-guiding function and fitted to the frame  10 . Therefore, the mobile phone can be provided with low cost without any special substrate or flexible board for compensating the distance between the key-top units  4  and  5  and the dome switches  8  on the board  6 . 
     6. The lengths of the key-top units  4  and  5  can be increased in the lateral direction of the casing while the tactile clicking sensation and the illuminating function of related-art button keys are maintained. Thus, the key-top units  4  and  5  can be pressed more easily. Furthermore, the outlines of the key-top units  4  and  5  match the outline of the touch panel module  13  provided on the front casing  1 , producing continuity therebetween. Thus, the value of the mobile phone in terms of design is increased. 
     Variation of the Embodiment 
     A variation of the mobile phone according to the above embodiment will now be described. While the mobile phone according to the above embodiment has two key-top units in total, specifically, the first key-top unit  4  and the second key-top unit  5 , a mobile phone according to the following variation has three or more key-top units. The number of key-top units to be provided is at least three and may be more. The following variation concerns a case where three key-top units in total, specifically, the first and second key-top units  4  and  5  with an intermediate key-top unit  80  interposed therebetween as shown in  FIG. 11 . 
       FIGS. 12A to 12E  are a front view, a top view, a bottom view, a right-side view, and a left-side view, respectively, of the intermediate key-top unit  80  included in the mobile phone according to the variation. As can be seen from  FIGS. 12A to 12E , the intermediate key-top unit  80  includes a projecting section  81  that is to project from the opening  14  when the intermediate key-top unit  80  is fitted into the opening  14 . The intermediate key-top unit  80  also includes flanges  82  projecting in opposite directions that are orthogonal to the longitudinal direction of the opening  14  so that the intermediate key-top unit  80  does not come off from the opening  14  when the projecting section  81  is made to project from the opening  14 . 
     Since the intermediate key-top unit  80  is interposed between the first and second key-top units  4  and  5 , ends  80   a  and  80   b  thereof are cut vertically in the direction in which the projecting section  81  projects so as not to produce gaps with respect to the end  20   b  of the first key-top unit  4  and the end  30   a  of the second key-top unit  5 . 
     The intermediate key-top unit  80  is provided on the bottom thereof, i.e., a surface thereof opposite the projecting section  81 , with an intermediate water-blocking sheet  83  bonded thereto. The intermediate water-blocking sheet  83  has a larger area than the bottom surface of the intermediate key-top unit  80 , and is made of a waterproof rubber material or the like processed into a substantially rectangular shape with a pair of securing tabs  84  and  85  at which the intermediate water-blocking sheet  83  is secured to the front casing  1  together with the intermediate key-top unit  80 . 
     As in the cases of the first and second key-top units  4  and  5  described above referring to  FIGS. 18A to 18F , the intermediate water-blocking sheet  83  has H-shaped sections with peripheral edges thereof projecting upward and downward. Therefore, when the mobile phone is assembled, the edges of the intermediate water-blocking sheet  83  come into close contact with the front casing  1  and the frame  10 . Thus, water and dust are prevented from entering the interior of the casing through gaps between the intermediate key-top unit  80  and the key-top units  4  and  5  and between the intermediate key-top unit  80  and the edge of the opening  14 . 
     Furthermore, as shown in  FIG. 12C , the intermediate water-blocking sheet  83  has shading treatment done with, for example, black ink along the periphery thereof. With the shading treatment, light from a corresponding one of the LEDs  7  is efficiently guided toward the intermediate key-top unit  80 . 
     Furthermore, the intermediate water-blocking sheet  83  has overlapping portions  86  and  87  that overlap a portion of the first water-blocking sheet  25  and a portion of the second water-blocking sheet  35 , respectively, when the intermediate key-top unit  80  is fitted into the opening  14  together with the first and second key-top units  4  and  5 . 
     In the mobile phone according to the embodiment described above, the overlapping portion  38  is provided on the second key-top unit  5 . Instead of the overlapping portion  38  provided on the second key-top unit  5 , the intermediate key-top unit  80  included in the mobile phone according to the variation has the overlapping portions  86  and  87  that overlap a portion of the first water-blocking sheet  25  and a portion of the second water-blocking sheet  35 , respectively. 
     The intermediate key-top unit  80  is shaped such that, in the longitudinal direction of the opening  14 , the sum of the lengths of the first, second, and intermediate key-top units  4 ,  5 , and  80  becomes substantially the same as the length of the opening  14 . 
     In this variation, the first, second, and intermediate key-top units  4 ,  5 , and  80  function as, for example, an off-hook key, an on-hook key, and a general-menu-displaying key, respectively. The first and second key-top units  4  and  5  have substantially the same length. The intermediate key-top unit  80  provided between the key-top units  4  and  5  is a little longer than the key-top units  4  and  5 . 
     In the mobile phone according to the variation, a pusher unit to be mounted on the pusher-unit mount  12  provided on the frame  10  shown in  FIG. 13  includes, referring now to  FIG. 14 , a connecting bar  90  having a substantially stick-like shape, a first pusher member  92 , and a second pusher member  94 . The first pusher member  92  has a first projection  91  and is to be positioned between the first key-top unit  4  and the dome switch  8  corresponding thereto. When the first key-top unit  4  is pressed, the pressing force is transmitted to the dome switch  8  through the first projection  91 . The second pusher member  94  has a second projection  93  and is to be positioned between the second key-top unit  5  and the dome switch  8  corresponding thereto. When the second key-top unit  5  is pressed, the pressing force is transmitted to the dome switch  8  through the second projection  93 . 
     In the mobile phone according to the variation, the pusher unit also includes an intermediate pusher member  96  having an intermediate projection  95 . The intermediate pusher member  96  is to be positioned between the intermediate key-top unit  80  and the dome switch  8  corresponding thereto. When the intermediate key-top unit  80  is pressed, the pressing force is transmitted to the dome switch  8  through the intermediate projection  95 . 
     The pusher unit also includes a first supporting portion  97 , a second supporting portion  98 , an intermediate supporting portion  99 , and a pair of locking hooks  100 . The first supporting portion  97  connects the first pusher member  92  to the connecting bar  90  and bends in response to the pressing of the first key-top unit  4 . The second supporting portion  98  connects the second pusher member  94  to the connecting bar  90  and bends in response to the pressing of the second key-top unit  5 . The intermediate supporting portion  99  connects the intermediate pusher member  96  to the connecting bar  90  and bends in response to the pressing of the intermediate key-top unit  80 . The locking hooks  100  are provided on the connecting bar  90  and secure the pusher unit to the frame  10 . 
     The pusher unit is formed such that the connecting bar  90 , the first pusher member  92 , the first supporting portion  97 , the second pusher member  94 , the second supporting portion  98 , the intermediate pusher member  96 , and the intermediate supporting portion  99  are integrally molded from, for example, a transparent or translucent light-conducting material, such as polycarbonate resin, that bends when a force is applied thereto and restores the original shape when the applied force is removed. 
       FIGS. 15A to 15D  show a process of fitting the key-top units  4 ,  5 , and  80  to the front casing  1 .  FIGS. 15A to 15D  are sectional views of the front casing  1  taken along the opening  14 . 
     To fit the first key-top unit  4  to the front casing  1 , referring to  FIG. 15A , the projecting section  20  of the first key-top unit  4  is inserted into the opening  14  from the back of the front casing  1 . Thus, the projecting section  20  of the first key-top unit  4  projects from the opening  14 , with the flanges  22  of the first key-top unit  4  being caught at the opening  14 . In this state, the first key-top unit  4  is slid toward the strength-maintaining projection  15   a  of the front casing  1  so that the extended portion  21  of the first key-top unit  4  is positioned above the strength-maintaining projection  15   a.    
     Subsequently, referring to  FIG. 15B , the projecting section  30  of the second key-top unit  5  is inserted into the opening  14  from the back of the front casing  1 . Thus, the projecting section  30  of the second key-top unit  5  projects from the opening  14 , with the flanges  32  of the second key-top unit  5  being caught at the opening  14 . In this state, the second key-top unit  5  is slid toward the strength-maintaining projection  15   b  of the front casing  1  so that the extended portion  31  of the second key-top unit  5  is positioned above the strength-maintaining projection  15   b.    
     Subsequently, referring to  FIG. 15C , the projecting section  81  of the intermediate key-top unit  80  is inserted into a space in the opening  14  between the first and second key-top units  4  and  5  from the back of the front casing  1 . Thus, the projecting section  81  of the intermediate key-top unit  80  projects from the opening  14 , with the flanges  82  of the intermediate key-top unit  80  being caught at the opening  14 . In this manner, as shown in  FIG. 15D , the intermediate key-top unit  80  is interposed between the first and second key-top units  4  and  5 . 
     When the first key-top unit  4  is slid toward the strength-maintaining projection  15   a  of the front casing  1  such that the extended portion  21  of the first key-top unit  4  is positioned above the strength-maintaining projection  15   a ; the second key-top unit  5  is then slid toward the strength-maintaining projection  15   b  of the front casing  1  such that the extended portion  31  of the second key-top unit  5  is positioned above the strength-maintaining projection  15   b  ; and the intermediate key-top unit  80  is then interposed between the key-top units  4  and  5 , referring now to  FIG. 16 , the pair of securing tabs  26  and  27  of the first water-blocking sheet  25 , the pair of securing tabs  36  and  37  of the second water-blocking sheet  35 , and the pair of securing tabs  84  and  85  of the intermediate water-blocking sheet  83  engage with projections  50 , respectively, provided on the front casing  1 . Thus, the key-top units  4 ,  5 , and  80  are secured to the front casing  1  at the opening  14 . 
     The opening  14  of the front casing  1  is provided by linearly and continuously cutting the front casing  1  from a position near the end  1   a  to a position near the end  1   b  of the front casing  1 . Furthermore, the ends  1   a  and  1   b  of the front casing  1  that define the opening  14  are reinforced with the pair of strength-maintaining projections  15   a  and  15   b.    
     Therefore, the opening  14  can be widened to extend from a position near the end  1   a  to a position near the end  1   b  of the front casing  1  while sufficient strength is provided to the ends  1   a  and  1   b  of the front casing  1  that define the opening  14 . Consequently, the outside dimensions of the key-top units  4 ,  5 , and  80  to be provided in the opening  14  can be increased. In particular, the outside dimensions of the first and second key-top units  4  and  5  to be positioned on both sides of the front casing  1  can be maximized such that the key-top units  4  and  5  reach near the ends  1   a  and  1   b  of the front casing  1 . 
     In the mobile phone according to the variation, the allowance for such increase in the outside dimensions of the key-top units  4 ,  5 , and  80  contributes to the flexible designs of the key-top units  4 ,  5 , and  80  and the casing. 
       FIG. 17A  is a side view of the key-top units  4 ,  5 , and  80  fitted in the opening  14  of the front casing  1 .  FIG. 17B  is a bottom view of the key-top units  4 ,  5 , and  80  fitted in the opening  14  of the front casing  1 . As can be seen from  FIGS. 17A and 17B , when the key-top units  4 ,  5 , and  80  are fitted in the opening  14  of the front casing  1 , the overlapping portions  86  and  87  of the intermediate water-blocking sheet  83  provided on the intermediate key-top unit  80  overlap a portion of the first water-blocking sheet  25  provided on the first key-top unit  4  and a portion of the second water-blocking sheet  35  provided on the second key-top unit  5 , respectively. Pressing portions  105  and  106 , shown in  FIG. 13  and described below, provided on the frame  10  come into contact with the overlapping portions  86  and  87 , respectively. 
     A process of mounting the pusher unit onto the frame  10  will now be described. Referring to  FIG. 13 , the frame  10  has the pusher-unit mount  12  including a groove  107  that receives the connecting bar  90  of the pusher unit, described above referring to  FIG. 14 , a concave-shaped first-pusher-member-receiving portion  108  that receives the first pusher member  92 , a concave-shaped second-pusher-member-receiving portion  109  that receives the second pusher member  94 , and a concave-shaped intermediate-pusher-member-receiving portion  110  that receives the intermediate pusher member  96 . 
     The first-pusher-member-receiving portion  108  has a hole  111  into which the first projection  91  of the first pusher member  92  is to be inserted. The second-pusher-member-receiving portion  109  has a hole  112  into which the second projection  93  of the second pusher member  94  is to be inserted. The intermediate-pusher-member-receiving portion  110  has a hole  113  into which the intermediate projection  95  of the intermediate pusher member  96  is to be inserted. 
     The groove  107  is provided with locking holes (not shown) to which the pair of locking hooks  100  of the pusher unit shown in  FIG. 14  are to be hooked, respectively. 
     To mount the pusher unit onto the pusher-unit mount  12 , the connecting bar  90  of the pusher unit is fitted into the groove  107 , the first projection  91  of the first pusher member  92  is then inserted into the hole  111  of the first-pusher-member-receiving portion  108 , and the first pusher member  92  is then fitted into the first-pusher-member-receiving portion  108 . Meanwhile, the second projection  93  of the second pusher member  94  is inserted into the hole  112  of the second-pusher-member-receiving portion  109 , and the second pusher member  94  is then fitted into the second-pusher-member-receiving portion  109 . Furthermore, the intermediate projection  95  of the intermediate pusher member  96  is inserted into the hole  113  of the intermediate-pusher-member-receiving portion  110 , and the intermediate pusher member  96  is then fitted into the intermediate-pusher-member-receiving portion  110 . 
     Thus, the pair of locking hooks  100  shown in  FIG. 14  are hooked into the locking holes provided in the groove  107 . Consequently, the pusher unit is secured to the pusher-unit mount  12  of the frame  10 . 
     To assemble the mobile phone, with the key-top units  4 ,  5 , and  80  fitted to the front casing  1  and the pusher unit mounted on the pusher-unit mount  12  of the frame  10 , as described above, the frame  10  and the board  6  are housed together in the casing formed by the front casing  1  and the rear casing  2 , shown in  FIG. 13 , and the back cover  3  is then put to the rear casing  2 . Thus, the mobile phone according to the variation is assembled. 
     As shown in  FIG. 13 , a portion of the frame  10  between the first-pusher-member-receiving portion  108  and the intermediate-pusher-member-receiving portion  110  forms the pressing portion  105 , which has a convex shape. Furthermore, a portion of the frame  10  between the second-pusher-member-receiving portion  109  and the intermediate-pusher-member-receiving portion  110  forms the pressing portion  106 , which has a convex shape. When the frame  10  is placed inside the casing formed by the front casing  1  and the rear casing  2 , the pressing portions  105  and  106  come into contact with the overlapping portions  86  and  87 , respectively, of the intermediate water-blocking sheet  83  provided on the intermediate key-top unit  80  fitted to the front casing  1 , thereby pressing the overlapping portions  86  and  87  with specific pressures. 
     The water-blocking sheets  25 ,  35 , and  83  provided on the respective key-top units  4 ,  5 , and  80  each have the peripheral edges thereof projecting upward and downward, as described above, thereby having H-shaped longitudinal and lateral sections. Therefore, as described above referring to  FIGS. 19 and 20 , the water-blocking sheets  25 ,  35 , and  83  come into close contact with the front casing  1  at the edges thereof projecting upward and with the frame  10  at the edges thereof projecting downward. 
     In the state where the key-top units  4 ,  5 , and  80  are fitted in the opening  14  next to one another, small gaps  71  shown in  FIG. 10  are produced between the key-top units  4  and  80  and between the key-top units  5  and  80 , and other small gaps shown in  FIGS. 19 and 20  are produced between the edge of the opening  14  and the peripheries of the key-top units  4 ,  5 , and  80 . These gaps may allow water and dust to enter the interior of the casing. 
     In the mobile phone according to the variation of the embodiment, however, since the H-shaped edges of the water-blocking sheets  25 ,  35 , and  83  provided on the key-top units  4 ,  5 , and  80  are in close contact with the front casing  1  and the frame  10  in such a manner as to surround the key-top units  4 ,  5 , and  80 , respectively, water and dust are prevented from entering the interior of the casing through the gap between the edge of the opening  14  and the key-top units  4 ,  5 , and  80 . 
     Furthermore, since the overlapping portions  86  and  87  where the intermediate water-blocking sheet  83  provided on the intermediate key-top unit  80  overlap the water-blocking sheets  25  and  35  of the key-top units  4  and  5  at the positions corresponding to the gaps  71  between the intermediate key-top unit  80  and the key-top units  4  and  5 , water and dust are prevented from entering the interior of the casing through the gaps  71 . Moreover, the convex-shaped pressing portions  105  and  106  of the frame  10  that are in contact with and pressed against the overlapping portions  86  and  87  enhance the prevention of entrance of water and dust into the interior of the casing through the gaps  71 . 
     In addition, elastic sealing members made of a rubber material or the like may be provided in the gaps provided between the extended portions  21  and  31  of the key-top units  4  and  5  and the respective strength-maintaining projections  15   a  and  15   b  of the front casing  1  when the key-top units  4  and  5  are fitted in the opening  14 . This further enhances the prevention of entrance of water and dust into the interior of the casing through the gaps. 
     The convex-shaped pressing portions  105  and  106  provided between the first- and intermediate-pusher-member-receiving portions  108  and  110  and between the second- and intermediate-pusher-member-receiving portions  109  and  110 , respectively, of the frame  10  according to the variation may alternatively be provided on the board  6  or the rear casing  2 , for example. That is, the pressing portions  105  and  106  may be provided at any position on the casing, except the front casing  1 , as long as the pressing portions  105  and  106  have convex shapes that press the overlapping portions  86  and  87 , respectively. 
     In the mobile phone according to the variation, the key-top units  4 ,  5 , and  80  function as, for example, an off-hook phone key, an on-hook phone key, and a general-menu-displaying key, respectively. When the first key-top unit  4  is pressed, referring to  FIG. 10 , the pressing force applied to the first key-top unit  4  is transmitted to the first pusher member  92 ; the first supporting portion  97  connecting the first pusher member  92  to the connecting bar  90  bends; and the first projection  91  presses the corresponding dome switch  8  on the board  6 . Thus, an off-hook phone operation is performed. 
     When the second key-top unit  5  is pressed, referring also to  FIG. 10 , the pressing force applied to the second key-top unit  5  is transmitted to the second pusher member  94 ; the second supporting portion  98  connecting the second pusher member  94  to the connecting bar  90  bends; and the second projection  93  presses the corresponding dome switch  8  on the board  6 . Thus, an on-hook phone operation is performed. 
     When the intermediate key-top unit  80  is pressed, referring also to  FIG. 10 , the pressing force applied to the intermediate key-top unit  80  is transmitted to the intermediate pusher member  96 ; the intermediate supporting portion  99  connecting the intermediate pusher member  96  to the connecting bar  90  bends; and the intermediate projection  95  presses the corresponding dome switch  8  on the board  6 . Thus, the general menu of the mobile phone is made to appear or disappear. 
     Key tops are often decorated with coating, printing, and the like for improved appearances thereof. In addition to such decoration, key tops are often provided with icons, such as characters, numbers, and symbols, representing individual operations. Furthermore, to help operations in dark environment, key tops are in general made of a material capable of guiding light from LEDs provided on a board or the like housed in a casing. Thus, for each key top, a contrast of illumination is produced between the icon and the other part, providing sufficient viewability of the icon. 
     In a case, however, where there is a substantial distance in the thickness direction of a casing between a board, having LEDs and housed in the casing, and key tops, it is difficult to form pusher structures that realize direct turning on/off of the dome switches on the board by using the key tops alone. Moreover, the illumination by the LEDs mounted on the board for illumination of the key tops may become unstable. 
     To solve such problems, the mobile phone according to the variation of the embodiment includes the pusher unit having a pusher function and a light-guiding function is fitted to the frame  10 , as described above referring to  FIG. 14 . 
     Specifically, the first, second, and intermediate pusher members  92 ,  94 , and  96  of the pusher unit shown in  FIG. 14  have bearing surfaces  92   a ,  94   a , and  96   a  that receive the key-top units  4 ,  5 , and  80 , respectively. Thus, the pusher unit provides a pusher structure that transmits pressing forces applied to the key-top units  4 ,  5 , and  80  to the dome switches  8  through the first, second, and intermediate projections  91 ,  93 , and  95 , respectively. 
     In the pusher structure, the first, second, and intermediate supporting portions  97 ,  98 , and  99  are connected to the connecting bar  90  in a so-called cantilever manner. This allows the first, second, and intermediate supporting portions  97 ,  98 , and  99  to bend when the key-top units  4 ,  5 , and  80  are pressed, respectively. Thus, the pressing forces are transmitted to the dome switches  8 , realizing on/off operations of the dome switches  8 . 
     In addition, the pusher unit is made of a transparent or translucent material suitable for guiding light, and has bevels  75 , shown in  FIG. 10 , angled at specific degrees so as to guide and diffuse light from the LEDs  7  toward the key-top units  4 ,  5 , and  80 . Therefore, the light from the LEDs  7  is evenly and stably guided and diffused toward the key-top units  4 ,  5 , and  80 . 
     With the pusher unit having a pusher function and a light-guiding function and fitted to the frame  10 , the mobile phone according to the variation of the embodiment provides a pusher structure that realizes direct turning on/off of the dome switches  8  on the board  6  even if there is a substantial distance in the thickness direction of the casing between the board  6 , having the LEDs  7  and housed in the casing, and the key-top units  4 ,  5 , and  80 . Furthermore, the mobile phone according to the variation realizes a light-guiding function of evenly and stably guiding and diffusing the light from the LEDs  7  on the board  6  toward the key-top units  4 ,  5 , and  80 . 
     Advantageous Effects of the Variation of the Embodiment 
     As is obvious from the above description, the mobile phone according to the variation of the embodiment provides the following advantageous effects. 
     1. With a resin connecting structure including the extended portions  21  and  31  of the first and second key-top units  4  and  5  and the pair of strength-maintaining projections  15   a  and  15   b  at the ends  1   a  and  1   b  of the front casing  1  defining the opening  14 , a sufficient structural base and strength of the casing are provided. 
     2. If an integral structure in which the first, second, and intermediate key-top units  4 ,  5 , and  80  are bonded to a single water-blocking sheet is employed, it is difficult to fit the key-top units  4 ,  5 , and  80  into the front casing  1  from the back of the front casing  1  because the extended portions  21  and  31  of the key-top units  4  and  5  interfere with the strength-maintaining projections  15   a  and  15   b , respectively. In the mobile phone according to the variation of the embodiment, however, the key-top units  4 ,  5 , and  80  are separate components, and it is therefore easy to fit the key-top units  4 ,  5 , and  80  into the opening  14  of the front casing  1  having the strength-maintaining projections  15   a  and  15   b.    
     3. The substantial distance in the thickness direction of the casing between the key-top units  4 ,  5 , and  80  and the dome switches  8  on the board  6  makes it difficult to provide sufficient waterproofness and dustproofness by using the key-top units  4 ,  5 , and  80  alone. In the mobile phone according to the variation of the embodiment, however, when the mobile phone is assembled by fitting the key-top units  4 ,  5 , and  80  into the opening  14 , the edges (the upward and downward projections forming the H-shaped sections) of the water-blocking sheets  25 ,  35 , and  83  provided on the respective key-top units  4 ,  5 , and  80  come into close contact with the front casing  1  and the frame  10 . This forms a closed sealing structure. Thus, water and dust are prevented from entering the interior of the casing through the gap between the edge of the opening  14  and the key-top units  4 ,  5 , and  80 , whereby sufficient waterproofness and dustproofness are provided. 
     4. With the overlapping portions  86  and  87  where the intermediate water-blocking sheet  83  provided on the intermediate key-top unit  80  overlaps the first and second water-blocking sheets  25  and  35  of the first and second key-top units  4  and  5  at the position corresponding to the gaps  71  between the intermediate key-top unit  80  and the key-top units  4  and  5 , water and dust are prevented from entering the interior of the casing through the gaps  71 . Moreover, the convex-shaped pressing portions  105  and  106  of the frame  10  that are in contact with and press the overlapping portions  86  and  87  enhance the prevention of entrance of water and dust into the interior of the casing through the gaps  71 . 
     5. With the substantial distance in the thickness direction of the casing between the key-top units  4 ,  5 , and  80  and the dome switches  8  on the board  6 , it is difficult to realize direct on/off operations of the dome switches  8  on the board  6  and to guide the light from the LEDs  7  by using the key-top units  4 ,  5 , and  80  alone. In the mobile phone according to the variation of the embodiment, however, a pusher structure and a light-guiding structure are realized with a single component, i.e., the pusher unit having a pusher function and a light-guiding function and fitted to the frame  10 . Therefore, the mobile phone can be provided with low cost without any special substrate or flexible board for compensating the distance between the key-top units  4 ,  5 , and  80  and the dome switches  8  on the board  6 . 
     6. The lengths of the key-top units  4 ,  5 , and  80  can be increased in the lateral direction of the casing while the tactile clicking sensation and the illuminating function of related-art button keys are maintained. Thus, the key-top units  4 ,  5 , and  80  can be pressed more easily. Furthermore, the outlines of the key-top units  4 ,  5 , and  80  match the outline of the touch panel module  13  provided on the front casing  1 , producing continuity therebetween. Thus, the value of the mobile phone in terms of design is increased. 
     Other Variation 
     While the embodiment and the variation thereof described above concern the case where the present invention is applied to a mobile phone, the present invention may also be applied to other electronic apparatuses having press-to-operate button keys, such as PHSs, PDAs, portable game machines, digital cameras, moving-image-capturing apparatuses, and notebook and desktop personal computers. In any case, advantageous effects as in the embodiment and the variation are provided. 
     The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2009-223529 filed in the Japan Patent Office on Sep. 28, 2009, the entire content of which is hereby incorporated by reference. 
     Since the embodiment and the variation thereof described above are only examples of the present invention, the present invention is not limited thereto, and various modifications can be made thereto depending on design requirements and other factors insofar as they are within the technical scope of the present invention.