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TECHNICAL FIELD 
     The present invention relates to a vehicle door provided with a main sash and an auxiliary sash. 
     BACKGROUND ART 
     A sash is provided to the upper section of a door body section of a vehicle door. The sash is a member capable of supporting a closed window panel, and comprises a main sash constituting the outer frame of a door, and an auxiliary sash provided in auxiliary fashion to a portion of the main sash. Patent Literature 1 is prior art related to such a vehicle door. 
     In a vehicle door such as that indicated in Patent Literature 1, a main sash and auxiliary sash are provided above the door body. The auxiliary sash is mounted so that the upper end abuts the main sash and is open in a substantially U-shaped (substantially C-shaped) configuration toward an openable/closable window panel. In other words, the auxiliary sash includes a vehicle interior wall on the vehicle interior side, a vehicle exterior wall on the vehicle exterior side, and a bottom wall that connects the vehicle interior wall and the vehicle exterior wall. Notches are formed in the upper ends of the U-shaped corners. In other words, a notch is formed in the upper end of the boundary between the vehicle interior wall and the bottom wall, and in the upper end of the boundary between the vehicle exterior wall and the bottom wall. These notches are formed to facilitate cutting for abutting the auxiliary sash against the main sash. A run channel for supporting the window panel is fitted into the substantially U-shaped auxiliary sash. 
     However, when water has made contact with the vehicle door, some of the droplets that have migrated along the door panel or the like may penetrate to the inner surface side of the substantially U-shaped auxiliary sash from the notches. The droplets that have penetrated to the inner surface side may seep out from between the run channel and the auxiliary sash to the distal end of the vehicle exterior wall or the vehicle interior wall. In particular, the vehicle interior wall is a location near the passenger compartment, and water seepage must be more reliably suppressed. 
     At this point, it is possible to consider disposing a sealing member so as to surround the notch on the vehicle interior side, or using a run channel with good sealing characteristics. However, when such countermeasures are used, component costs are increased and the cost of the vehicle door overall is increased. 
     PRIOR ART LITERATURE 
     Patent Literature 1: JP 2007-176193 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide a vehicle door capable of suppressing water seepage from the vehicle interior wall of the auxiliary sash with minimal increase in the component cost. 
     Solution to Problem 
     In accordance with the present invention, there is provided a vehicle door comprising: a main sash disposed above a door body and surrounding a window opening, and an auxiliary sash with an upper end abutting the main sash, characterized in that: the main sash is provided with a U-shaped cross-section part that opens in a substantially U-shaped configuration toward the window opening; the U-shaped cross-section part comprises a first vehicle interior wall on the vehicle interior side, a first vehicle exterior wall on the vehicle exterior side, and a first bottom wall that connects the first vehicle interior wall and the first vehicle exterior wall; the auxiliary sash comprises a second vehicle interior wall abutting the first vehicle interior wall, a second vehicle exterior wall abutting the first vehicle exterior wall, and a second bottom wall that connects the second vehicle interior wall and the second vehicle exterior wall; a notch cut out along the first vehicle exterior wall is formed solely in the second vehicle exterior wall among the second vehicle interior wall and the second vehicle exterior wall; a top side of the second vehicle interior wall is provided contiguously to a top side of the second bottom wall; and the lower end of the notch is positioned below the top side of the second vehicle interior wall and the top side of the second bottom wall. 
     Preferably, a second notch cut out along the second vehicle interior wall is formed in the first vehicle interior wall. 
     It is preferable that a reinforcement member be mounted to the surface of the first vehicle interior wall on the vehicle interior side, a cover section extending toward the second vehicle interior wall be provided to the reinforcement member, and a contact section between the first vehicle interior wall and the second vehicle interior wall be covered by the cover section. 
     Preferably, the cover section is offset toward the vehicle interior with respect to a connecting section in which the reinforcement member is connected to the first vehicle interior wall. 
     It is preferable that a run channel capable of supporting a window panel be mounted on the U-shaped cross-section part, the run channel be provided with a vehicle interior lip bent back to the first vehicle interior wall on the vehicle interior side, and the lower end of the vehicle interior lip align with the contact section. 
     It is preferable that a corner garnish for covering the contact section from the vehicle interior side be provided to the surface of the cover section on the vehicle interior side, and the lower end of the vehicle interior lip align with the upper end of the corner garnish. 
     Preferably, a mounting section for mounting the auxiliary sash to the door body is provided to the second vehicle interior wall. 
     It is preferable that the second vehicle interior wall be offset toward the vehicle exterior side relative to the first vehicle interior wall, and the mounting section be mounted to the reinforcement member by a fastening member. 
     Advantageous Effects of Invention 
     In the present invention, the top side of the second vehicle interior wall (the vehicle interior wall of the auxiliary sash) is provided contiguously to the top side of the second bottom wall (the bottom wall of the auxiliary sash). Additionally, the lower end of the notch formed solely in the second vehicle exterior wall (vehicle exterior wall of the auxiliary sash) is positioned below the top side of the second vehicle interior wall and the top side of the second bottom wall. Water may penetrate from the gap between the first vehicle exterior wall and the second vehicle exterior wall and may migrate around the reverse surface of the second bottom wall. The top side of the second bottom wall and the second vehicle interior wall are positioned above and the notch is not formed in the vehicle interior side, and it is therefore possible to inhibit the penetration of water to the inner surface side of the second vehicle interior wall. A component does not need to be newly added to inhibit the penetration of water, and the component cost can accordingly be kept from increasing. In other words, the vehicle door can be said to be capable of inhibiting water seepage from the vehicle interior wall of the auxiliary sash while keeping the component cost from increasing. 
     Furthermore, in the present invention, a second notch cut out along the second vehicle interior wall is formed in the first vehicle interior wall. In other words, a notch is formed in the vehicle interior wall on the main sash side. Forming a notch on the main sash side makes it possible to cause the auxiliary sash to reliably abut the main sash without the second vehicle interior wall being notched. 
     In the present invention, a reinforcement member is mounted to the surface of the first vehicle interior wall on the vehicle interior side. A cover section for covering a contact section between the first vehicle interior wall and the second vehicle interior wall is formed on the reinforcement member. The contact section between the first vehicle interior wall and the second vehicle interior wall is covered by the reinforcement member for reinforcing the first vehicle interior wall. The contact section can be covered by the single member, the reinforcement member, while the strength of the main sash is enhanced. 
     In the present invention, the cover section is offset toward the vehicle interior with respect to a connecting section in which the reinforcement member is connected to the first vehicle interior wall. The cover section and the second vehicle interior wall are thereby set apart from each other. Setting these components apart makes it possible to facilitate assembly of the auxiliary sash. 
     In the present invention, a run channel capable of supporting a window panel is mounted on the U-shaped cross-section part of the main sash. The lower end of the vehicle interior lip of the run channel aligns with the contact section between the first vehicle interior wall and the second vehicle interior wall. Since the cover section is offset toward the vehicle interior, when the vehicle interior lip of the run channel is provided as far as the position of the covering section, a problem arises in that the vehicle interior lip increases in size. Aligning the lower end of the vehicle interior lip with the contact section makes it possible to prevent having the vehicle interior lip increase in size and to prevent a step from forming against the lip on the connecting section side. 
     Additionally, since the lower end of the vehicle interior lip can be brought into contact with the cover section for assembling, positioning during assembly is facilitated and assembling is easier to perform. 
     In the present invention, a corner garnish for covering the cover section from the vehicle interior side is provided to the surface of the cover section on the vehicle interior side. Additionally, the lower end of the vehicle interior lip is aligned with the upper end of the corner garnish. Covering the cover section (reinforcement member) with the corner garnish makes it possible to increase the attractiveness of the external appearance. When viewed from the vehicle interior side, the vehicle interior lip is contiguously disposed with the corner garnish. This makes it possible to further increase the attractiveness of the external appearance. 
     In the present invention, a mounting section for mounting the auxiliary sash to the door body is provided to the second vehicle interior wall. A mounting section is provided to the second vehicle interior wall, which has high rigidity, without the formation of a notch. High mounting strength can thereby be obtained. 
     In the present invention, the second vehicle interior wall is offset toward the vehicle exterior relative to the first vehicle interior wall. Additionally, the mounting section is mounted to the reinforcement member by a fastening member. By having the second vehicle interior wall offset toward the vehicle exterior relative to the first vehicle interior wall, the reinforcement member and the auxiliary sash can be set apart from each other. It is thereby possible to facilitate assembly of the auxiliary sash. Also, a mounting section can be disposed in the gap formed by this offset arrangement. For this reason, the reinforcement member and the auxiliary sash can be securely fastened by the fastening member. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a front view of the vehicle door according to an embodiment of the present invention; 
         FIG. 2  is a front view with the outer panel removed from the vehicle door shown in  FIG. 1 ; 
         FIGS. 3A-3B  are views illustrating mounting of the inner panel, the reinforcing member, the main sash, and the auxiliary sash shown in  FIG. 2 ; 
         FIG. 4  is a view taken in the direction of arrow  4  of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 4 ; and 
         FIG. 6  is a view taken in the direction of arrow  6  of  FIG. 4 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention are described below with reference to the accompanying drawings. In the description, left and right indicate left and right with reference to an occupant of a vehicle in which the vehicle door is mounted, and front and rear indicate front and rear with reference to the travel direction of the vehicle. In the drawings, Fr indicates front, Rr indicates rear, Ce indicates lateral center, Up indicates up, and Dw indicates down. 
     &lt;Embodiments&gt; 
     Reference will be made to  FIGS. 1 and 2 . A door mounted on the rear right side of a vehicle body is shown as an example of a vehicle door  10 . The vehicle door  10  includes a door body  20  swingably mounted on the vehicle body, a main sash  30  in a U shape (substantially C shape) facing downward disposed above the door body  20 , an auxiliary sash  50  abutting the rear lower end of main sash  30 , and a window panel  13  retractable toward a window opening  12  surrounded by the main sash  30  and the auxiliary sash  50 . 
     The door body  20  is composed of an outer panel  21  on the vehicle exterior side and an inner panel  22  on the vehicle interior side, one superimposed on the other. The door body  20  is formed in a closed sectional shape by superimposition of the two panels. 
     In particular, the main sash  30  is composed of a plurality of contiguously disposed members, as shown in  FIG. 1 . More specifically, the main sash  30  includes a front edge section  31  extending along the end section of the front side of the door body  20 , a front corner section  32  disposed in a substantially L-shaped configuration along the corner of the window opening  12  from the upper end of the front edge section  31 , an upper edge section  33  extending rearward along the upper edge of the window opening  12  from the rear end of the front corner section  32 , a rear corner section  34  disposed in an L shape along the corner of the window opening  12  from the rear end of the upper edge section  33 , and a rear edge section  40  extending along the rear end of the door body  20  from the lower end of the rear corner section  34 . 
     The auxiliary sash  50  is a roll-formed article extending toward the rear edge section  40  of the main sash  30 . The auxiliary sash  50  is disposed substantially parallel to the front edge section  31  along the direction of retraction (see arrow) of the window panel  13 . 
     Reference will be made to  FIG. 3A . A reinforcement member  60  is joined to the inner panel  22 . The lower end of the rear edge section  40  is superimposed on the reinforcement member  60  as viewed from the front face of the vehicle door  10  (the vehicle body, with the vehicle door  10  mounted, viewed from the side face). In other words, the inner panel  22 , the reinforcement member  60 , and the rear edge section  40  (main sash  30 ) are superimposed in the stated sequence from the vehicle interior side toward the vehicle exterior side. The reinforcement member  60  is a steel member formed by press-forming. 
     Referring additionally to  FIG. 3B , when the vehicle door  10  is viewed from the front surface, a contact section P 1  in which the upper end of the auxiliary sash  50  makes contact against the rear edge section  40  of the main sash  30  is superimposed on the reinforcement member  60 . In other words, the upper end of the auxiliary sash  50  abuts the rear edge section  40  of the main sash  30  on the reinforcement member  60 . Expressed differently, the inner panel  22 , the reinforcement member  60 , and the auxiliary sash  50  are superimposed in the stated sequence from the vehicle interior side toward the vehicle exterior side. 
     A separate mounting bracket  14  is joined to the lower part of the auxiliary sash  50 . The auxiliary sash  50  is mounted to the inner panel  22  via the mounting bracket  14  and a later-described mounting section. 
     Reference will be made to  FIGS. 4 and 5 . A corner garnish  70  is mounted as a decorative component on the surface of the inner panel  22  on the vehicle interior side. The corner garnish  70  includes a base  71  disposed along the surface of the inner panel  22  on the vehicle interior side, and an extension  72  extending from the front end of the base  71  forward toward the vehicle exterior side. A pawl  73  formed on the reverse surface of the base  71  is latched to the inner panel  22 . 
     An outer panel bend section  23  that includes a first bend section  23   a  bent from a general surface section  21   a  of the outer panel  21  toward the reinforcement member  60 , and a second bend section  23   b  bent from the distal end of the first bend section  23   a  along the surface of the reinforcement member  60  on vehicle exterior side. It can also be said that the outer panel bend section  23  is formed in a substantially L-shaped configuration from the general surface section  21   a  of the outer panel  21  toward the vehicle interior. 
     The rear edge section  40  (main sash  30 ) is provided with a U-shaped cross-section part  41  (C-shaped cross section  41 ) that opens in a substantially U-shaped (substantially C-shaped) configuration toward the window opening  12 . The U-shaped cross-section part  41  includes a first vehicle interior wall  42  on the vehicle interior side, a first vehicle exterior wall  43  on the vehicle exterior side, and a first bottom wall  44  that connects the first vehicle interior wall  42  and the first vehicle exterior wall  43 . In particular, a main-section run channel  15  (run channel  15 ) for supporting a window panel is fitted into the U-shaped cross-section part  41 , as indicated by the imaginary line in  FIG. 4 . A lip-shaped vehicle interior lip  15   a  bent back from the edge of the first vehicle interior wall  42  toward the vehicle interior side is formed on the main-section run channel  15 . 
     The auxiliary sash  50  includes a second vehicle interior wall  52  that abuts the first vehicle interior wall  42 , a second vehicle exterior wall  53  that abuts the first vehicle exterior wall  43 , and a second bottom wall  54  that connects the second vehicle interior wall  52  and the second vehicle exterior wall  53 . The top side  52   b  of the second vehicle interior wall  52  is contiguously provided to the top side  54   b  of the second bottom wall  54 . An auxiliary run channel  16  for supporting a window panel is fitted into the auxiliary sash  50 . 
     A mounting section  17  for mounting the auxiliary sash  50  to the inner panel  22  is joined to the surface of the vehicle interior side of the second vehicle interior wall  52  of the auxiliary sash  50 . A fastening member  18  is fastened to the mounting section  17  joined to the auxiliary sash  50 , whereby the outer panel  21  (outer panel bend section  23 ), the reinforcement member  60 , and the inner panel  22  are integrally fastened in the stated sequence from the vehicle exterior side. In other words, the auxiliary sash  50 , the outer panel  21 , the reinforcement member  60 , and the inner panel  22  are brought together in the mounting section  17 . 
     Referring to  FIG. 6  as well, a first notch  53   a  (notch  53   a ) cut out along the first vehicle exterior wall  43  is formed only in the second vehicle exterior wall  53  among the second vehicle interior wall  52  and the second vehicle exterior wall  53 . Also, a second notch  42   a  cut out along the second vehicle interior wall  52  is formed in the first vehicle interior wall  42 . A lower end  53   b  of the first notch  53   a  is positioned below a top side  52   b  of the second vehicle interior wall  52  and a top side  54   b  of the second bottom wall  54 . 
     A lower end  15   b  of the vehicle interior lip  15   a  is aligned with the contact section P 1  between the first vehicle interior wall  42  and the second vehicle interior wall  52 . In other words, the top side  54   b  of the second bottom wall  54 , the top side  52   b  of the second vehicle interior wall  52 , and the lower end  15   b  of the vehicle interior lip  15   a  are contiguous. 
     The reinforcement member  60  is mounted on the surface of the first vehicle interior wall  42  on the vehicle interior side. A cover section  61  extending at an incline toward the second vehicle interior wall  52  is formed on the other hand the reinforcement member  60 . The contact section P 1  is covered by the cover section  61 . Referring to  FIG. 3A  as well, the cover section  61  is offset toward the vehicle interior with respect to a connecting section  62  in which the reinforcement member  60  is connected to the first vehicle interior wall  42 . The boundary between the cover section  61  and the connecting section  62  is aligned with the portion in which the contact section P 1  is formed. 
     The cover section  61  is covered by the corner garnish  70  from the vehicle interior side. The lower end  15   b  of the vehicle interior lip  15   a  is covered by the corner garnish  70  from the vehicle interior side. 
     Returning to  FIG. 5 , water may penetrate from the gap between the first vehicle exterior wall  43  and the second vehicle exterior wall  53  and may migrate around the reverse surface of the second bottom wall  54 . Since a notch (first notch  53   a ) is formed only in the vehicle exterior side, it is possible to inhibit the penetration of water to the inner side of the second vehicle interior wall  52 . Since no components need be newly added to inhibit the penetration of water, the component cost can be kept from increasing. In other words, the vehicle door can  10  be regarded to be capable of inhibiting water seepage from the second vehicle interior wall  52  (the vehicle interior wall of the auxiliary sash  50 ) while keeping component cost from increasing. 
     Furthermore, the mounting section  17  for mounting the auxiliary sash  50  to the door body  20  (inner panel  22 ) is joined to the second vehicle interior wall  52 . The mounting section  17  is provided to the second vehicle interior wall  52 , which has high rigidity, without a notch being formed. High mounting strength can thereby be obtained. 
     Furthermore, the second vehicle interior wall  52  is offset toward the vehicle exterior relative to the first vehicle interior wall  42 . Additionally, the mounting section  17  is mounted on the reinforcement member  60  using the fastening member  18 . The second vehicle interior wall  52  being offset toward the vehicle exterior relative to the first vehicle interior wall  42  makes it possible to set the reinforcement member  60  and the auxiliary sash  50  apart from each other. The assembly characteristics of the auxiliary sash  50  can thereby be enhanced. 
     Furthermore, the mounting section  17  can be disposed in the gap formed by this offset arrangement. For this reason, the reinforcement member  60  and the auxiliary sash  50  can be securely fastened by the fastening member. 
     Referring to  FIG. 4  as well, the second notch  42   a  cut out along the second vehicle interior wall  52  is formed in the first vehicle interior wall  42 . Notching the main sash  30  side makes it possible to reliably abut the auxiliary sash  50  against the main sash  30  without a notch being formed in the second vehicle interior wall  52 . 
     Furthermore, the reinforcement member  60  is mounted on the surface of the first vehicle interior wall  42  on the vehicle interior side, and the cover section  61  for covering the contact section P 1  between the first vehicle interior wall  42  and the second vehicle interior wall  52  is formed on the reinforcement member  60 . The contact section P 1  between the first vehicle interior wall  42  and the second vehicle interior wall  52  is covered by the reinforcement member  60  for reinforcing the first vehicle interior wall  42 . The contact section P 1  can be covered while the strength of the main sash  30  is increased by a single member, i.e., the reinforcement member  60 . 
     Moreover, the cover section  61  is offset toward the vehicle interior with respect to the connecting section  62  in which the reinforcement member  60  is connected to the first vehicle interior wall  42 . The cover section  61  and the second vehicle interior wall  52  are thereby set apart from each other. This set-apart arrangement makes it possible to improve the assembly characteristics of the auxiliary sash  50 . 
     Additionally, the main-section run channel  15  capable of supporting the window panel  13  is mounted to the U-shaped cross-section part  41  of the main sash  30  (rear edge section  40 ), and the lower end  15   b  of the vehicle interior lip  15   a  of the main-section run channel  15  is aligned with the contact section P 1  between the first vehicle interior wall  42  and the second vehicle interior wall  52 . Since the cover section  61  is offset toward the vehicle interior, the size of the vehicle interior lip  15   a  is increased when the vehicle interior lip  15   a  of the main-section run channel  15  is provided as far as the position of the cover section  61 . Aligning the lower end  15   b  of the vehicle interior lip  15   a  with the contact section P 1  makes it possible to prevent the size of the vehicle interior lip  15   a  from increasing and to prevent a step from being formed with the lip on the connecting section  62  side. 
     Also, since the lower end  15   b  of the vehicle interior lip  15   a  can be assembled on the cover section  61  by being brought into contact therewith, positioning during assembly is facilitated and assembling is easier to perform. 
     Furthermore, the corner garnish  70  for covering the cover section  61  from the vehicle interior side is provided to the surface of the cover section  61  on the vehicle interior side, and the lower end  15   b  of the vehicle interior lip  15   a  is aligned with the upper end of the corner garnish  70 . Covering the cover section  61  (reinforcement member  60 ) with the corner garnish  70  makes it possible to increase the attractiveness of the external appearance. When viewed from the vehicle interior side, the vehicle interior lip  15   a  is contiguously disposed with the corner garnish  70 . This makes it possible to further increase the attractiveness of the external appearance. 
     The vehicle door according to the present invention was described using a rear vehicle door as an example, but application as a front vehicle door is also possible. In such a case, an auxiliary sash may be provided to the front end side, or other design modifications can be made, as appropriate, for application to a front vehicle door. 
     INDUSTRIAL APPLICABILITY 
     The vehicle door of the present invention is advantageous for an automobile. 
     REFERENCE SIGNS LIST 
     
         
           10 : Vehicle door 
           12 : Window opening 
           15 : Main-section run channel (run channel) 
           15   a : Vehicle interior lip 
           15   b : Lower end of the vehicle interior lip 
           17 : Mounting section 
           18 : Fastening member 
           20 : Door body 
           30 : Main sash 
           41 : U-shaped cross-section part 
           42 : First vehicle interior wall 
           42   a : Second notch 
           43 : First vehicle exterior wall 
           44 : First bottom wall 
           50 : Auxiliary sash 
           52 : Second vehicle interior wall 
           52   b : Top side (of the second vehicle interior wall) 
           53 : Second vehicle exterior wall 
           53   a : First notch (notch) 
           53   b : Lower end (of the first notch) 
           54 : Second bottom wall 
           54   b : Top side (of the second bottom wall) 
           60 : Reinforcement member 
           61 : Cover section 
           62 : Connecting section 
           70 : Corner garnish 
         P 1 : Contact section

Summary:
A vehicle door is provided with: a main sash that is disposed above a door main body and that encircles a window opening; and an auxiliary sash of which the upper end abuts the main sash. The auxiliary sash is provided with: a second vehicle interior wall abutting a first vehicle interior wall of the main sash; a second vehicle exterior wall abutting a first vehicle exterior wall; and a second bottom wall coupling the second vehicle interior wall and the second vehicle exterior wall. A notch that is notched alongside the first vehicle exterior wall is formed solely at the second vehicle exterior wall. The bottom end of the notch is disposed below the tip side of the second bottom wall and the top side of the second vehicle interior wall formed contiguously.