Abstract:
An information retrieving apparatus is advantageously used in a voice operation of a vehicle navigation system. The apparatus includes a storage unit for storing the plural pieces of the candidate information composing a hierarchical structure including m hierarchies (m is a natural number not less than 2), and the apparatus retrieves a target information from the hierarchical structure. The apparatus includes an input unit for inputting information, a judging unit for judging a correspondence of the inputted information with one of the plural pieces of the candidate information and for judging the hierarchy of the candidate information if the correspondence is obtained, and a retrieving unit for retrieving one of the plural pieces of the candidate information as a target information based on a judged result of the judging unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technical field of an information retrieval apparatus for retrieving information such as a destination spot in a navigation device which displays a travel route of a mobile object such as a vehicle, names of famous places around the travel route and the like as well as a map so as to assist the driving of the mobile object. 
     2. Description of Related Art 
     At present, there is known a navigation device, which displays a position mark (guide mark) representing a current position of a mobile object on a map so as to guide a travel route to a destination spot based on the displayed map, as a position measuring device used for various mobiles such as a vehicle, an aircraft and a vessel. In such navigation devices, vehicle navigation devices to be mounted on vehicles are roughly divided into two types: a stand-alone type navigation device and a GPS (Global Positioning System) type navigation device. 
     The stand-alone type navigation system obtains a travel direction and a travel distance of a vehicle by means of independent sensors such as a speed sensor and an angular velocity sensor provided on the vehicle, and then calculates a current position of the vehicle by successively adding the obtained travel direction and travel distance to a reference point so as to display a position mark and a corresponding map on a display screen based on the calculated current position. 
     The GPS type navigation system receives a position measuring radio waves from a plurality of GPS satellites launched into space via a GPS receiver, and calculates a current position of the vehicle by a three-dimensional measuring method or a two-dimensional measuring method based on the received result so as to display a position mark and a corresponding map on a display screen based on the calculated current position. 
     Further, a so-called hybrid-type vehicle navigation device having both functions of the above-mentioned stand-alone type and GPS types navigation systems is recently known. 
     In addition, among the above respective vehicle navigation devices, there is also known a navigation device for superposing a scheduled travel route, which the vehicle plans to follow, on a map to be displayed together with famous places around the travel route. 
     According to the above respective vehicle navigation devices, since a user (i.e., driver) can know a current position of the vehicle and a map around the current position in connection with each other, he or she can reach the destination without getting lost even if he or she goes to the area for the first time. 
     On the other hand, in the above-mentioned vehicle navigation device, there is a case, for example, that the driving is started after a destination spot is set and a travel route to the destination spot is determined in advance. At this time, in setting the destination spot, a plurality of destination candidates are arranged in a hierarchical data structure, and the particular destination spot is set by successively searching the hierarchical structure for that particular destination in an interactive manner with the user. 
     Namely, according to the vehicle navigation device having the destination spot setting function, when a destination spot is set concretely, plural types of destination attribute names (for example, attribute names representing types of destination spots such as a golf course, a theme park and a restaurant. The hierarchy of the destination candidates composed of the attribute names is the first hierarchy) are first displayed, and the user selects a target attribute of the destination spot from those plural attribute names. Then, plural names of prefectures (the hierarchy of the destination candidates composed of prefecture names is the second hierarchy) in which facilities including the target destination spot exist are displayed. When a name of the target prefecture where the destination spot exists is selected, plural names of areas in the target prefectures (for example, the names of areas such as northern part and southern part. The hierarchy of the spot candidates including the area names is the third hierarchy) are displayed so that a name of a target area where the destination spot exists is selected from the plural area names. Next, plural names of cities/towns/villages existing in the target area (the hierarchy of the spot candidates including the names of cities/towns/villages is the fourth hierarchy) are displayed, and a name of the target city/town/village where the destination spot exists is selected. Then, finally, plural names of concrete facilities having the target attribute selected previously in the selected city/town/village are displayed as destination candidates, and the destination spot is selected from those displayed names of the facilities and set. 
     However, in the case where the above-described destination spot setting function of the, vehicle navigation device is used, a driver needs to change the displayed prefecture names until a desired prefecture name is displayed in order to select the prefecture name included in the second hierarchy, for example. Therefore, there arose a problem that unnecessary time is required for searching the target prefecture name. 
     As for this problem, in the vehicle navigation device capable of inputting information by using a voice recognizing function which is being put into practical use recently, all candidate names of prefectures should be vocally reproduced by the device until a desired prefecture name comes out. As a result, the required time becomes unnecessarily long and the driving is hindered because many unnecessary voices are heard. 
     Further, according to the above-mentioned destination spot setting function, it is always necessary to start selecting candidates from the first hierarchy then down to the deeper hierarchies. In this case, for example, even if a user already knows a name of city/town/village where the destination spot exists (belonging to the fourth hierarchy), the driver should unnecessarily start the selecting operation from the uppermost hierarchy, and hence unnecessary operation should be made. 
     SUMMARY OF THE INVENTION 
     In view of the above, an object of the present invention is to provide an information retrieving apparatus and an information retrieving method, which are capable of retrieving a desired destination spot information quickly, accurately and conveniently in a navigation device having a destination spot setting function using spot candidates data of hierarchical structure. 
     According to one aspect of the present invention, there is provided an information retrieving apparatus for retrieving target information from plural pieces of candidate information, including: a storage unit for storing the plural pieces of the candidate information in a hierarchical structure including m hierarchies (m is a natural number not less than 2): an input unit for inputting information; a judging unit for judging a correspondence of the inputted information with one of the plural pieces of the candidate information and for judging the hierarchy of the candidate information if the correspondence is obtained; and a retrieving unit for retrieving one of the plural pieces of the candidate information as a target information based on a judged result of the judging unit. 
     In accordance with the apparatus thus configured, the storage unit stores the plural pieces of the candidate information in a hierarchical structure including m hierarchies. A user can input information via the input unit. Then, the judging unit judges a correspondence of the inputted information with one of the plural pieces of the candidate information, and then judges the hierarchy of the candidate information if the correspondence is obtained. Thereafter, the retrieving unit retrieves one of the plural pieces of the candidate information as a target information based on a judged result of the judging unit. 
     Therefore, since a hierarchy of inputted candidate information is judged first and then the information is retrieved based on the judged result, it is possible to retrieve target information in the order of hierarchies, or alternatively possible to retrieve target information while skipping one or plural hierarchies and jumping from a hierarchy to a hierarchy. Consequently, a desired target information can be retrieved conveniently in the information retrieval apparatus. 
     The hierarchical structure may be arranged such that each piece of the candidate information belonging to (n+1)-th hierarchy (n is a natural number and n&lt;m−1) is associated with one of the plural pieces of the candidate Information belonging to n-th hierarchy. With this arrangement, if candidate information in the upper hierarchy are different, candidate information in the lower hierarchy corresponding to the candidate information in the upper information are different. Therefore, the hierarchical structure can be constituted to include plural types of candidate information. 
     It is preferred that the information retrieving apparatus further includes an output unit for outputting input request information which requests a user to input an input information corresponding to the candidate information belonging to one hierarchy of the hierarchical structures, wherein the output unit includes: a unit for outputting the input request information requesting the input of the candidate information belonging to the (p+1)-th hierarchy if the candidate information belonging to the p-th hierarchy is inputted in response to the input request information requesting the input of the candidate information belonging to the p-th hierarchy (p is a natural number and p&lt;m−1); and a unit for outputting the input request information requesting the input of the candidate information belonging to the (p+q+1)-th hierarchy if the candidate information belonging to the (p+q)-th hierarchy is inputted in response to the input request information requesting the input of the candidate information belonging to the p-th hierarchy (q is a natural number and (p+q)&lt;m−1). This enables the retrieval in the order of the hierarchical structure or alternatively enables the retrieval with jumping between the hierarchies. 
     In addition, the output unit may further include a unit for outputting the input request information requesting the input of the candidate information in r-th hierarchy (r is a natural number and r&lt;m−1) if no candidate information is inputted for a predetermined time in response to input request information requesting the input of the candidate information belonging to the r-th hierarchy. With this arrangement, if no input in response to the input request information of a hierarchy is made, another input request information of the lower hierarchy is outputted, thereby accelerating the retrieval of the target information. 
     It a preferred embodiment, the candidate information may include a plurality of candidate information groups each including the plural pieces of the candidate information in one hierarchy, wherein the candidate information included in one candidate information group is different from the candidate information included in another candidate information groups, and wherein, if no input information corresponding to the candidate information is inputted for a predetermined time in response to input request information requesting the input of the candidate information included in one candidate information group, the output unit outputs the input request information requesting the input of the candidate information included in a candidate information group other than the one candidate information group. 
     With this arrangement, an output of one input request information can be rapidly made. In addition, the desired candidate information can be inputted without the need to output all candidate information in one hierarchy. 
     The output unit may output the input request information in a form of voice, wherein the information inputted through the input unit is voice information, and wherein the input unit includes a voice recognition unit for recognizing a content of the inputted voice information. This achieves safer and more convenient input of information. 
     According to another aspect of the present invention, there is provided an information retrieving apparatus for retrieving target information from plural pieces of candidate information, including: a storage unit for storing the plural pieces of the candidate information in a hierarchical structure including a plurality of hierarchies; an input unit for inputting information; a determining unit for determining the candidate information corresponding to the inputted information; and a retrieving unit for retrieving one of the plural pieces of candidate information which belong to the lower hierarchy of the determined candidate information and are associated with the determined candidate information. Consequently, a desired target information can be retrieved conveniently in the information retrieval apparatus. 
     According to still another aspect of the present invention, there is provided a method of retrieving target information from plural pieces of candidate information composing a hierarchical structure including m hierarchies (m is a natural number not less than 2), wherein the hierarchical structure is arranged such that each piece of the candidate information belonging to (n+1)-th hierarchy (n is a natural number and n&lt;m−1) is associated with one of the plural pieces of the candidate information included in n-th hierarchy, the method including the steps of: receiving voice information; judging a correspondence of inputted voice information with one, of the plural pieces of the candidate information and judging the hierarchy of the candidate information if the correspondence is obtained; and retrieving one of the plural pieces of the candidate information as a target information based on a judged result of the judging step. 
     In accordance with the apparatus thus configured, the plural pieces of the candidate information are stored in a hierarchical structure including m hierarchies. An input information is received, and then a correspondence of the inputted information with one of the plural pieces of the candidate information is judged. Then, the hierarchy of the candidate information is judged if the correspondence is obtained. Thereafter, one of the plural pieces of the candidate information is retrieved as a target information based on a judgement result. 
     Therefore, it is possible to retrieve target information in the order of hierarchies, or to retrieve target information while skipping one or plural hierarchies and jumping from a hierarchy to a hierarchy. 
     The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram showing the configuration of the vehicle navigation apparatus according to the present invention. 
     FIGS. 2A and 2B are schematic diagrams showing the hierarchical structure of request messages and recognition words, respectively. 
     FIG. 3 is a flowchart showing a main routine of information % retrieving process according to an embodiment of the present invention. 
     FIG. 4 is a flowchart showing a hierarchy judgement routine in the information retrieving process of the invention. 
     FIG. 5 is a flowchart showing a following part of the routine shown in FIG.  4 . 
     FIG. 6 is a flowchart showing a following part of the routine shown in FIG.  5 . 
     FIGS. 7A and 7B are flowcharts showing hierarchy determination routine in the information retrieving process of the invention. 
     FIG. 8 is another flowchart showing a following part of the routine shown in FIG.  5 . 
     FIG. 9 is still another flowchart showing a following part of the routine shown in FIG.  5 . 
     FIGS. 10A and 10B are flowcharts showing hierarchy determination routine in the information retrieving process of the invention. 
     FIG. 11 is still another flowchart showing a following part of the routine shown in FIG.  5 . 
     FIG. 12 is still another flowchart showing a following part of the routine shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the present invention will be described below. It is noted that the embodiment described below is directed to the case where the present invention is applied to a vehicle navigation device which has a voice recognition function and a voice synthesizing function and is capable of inputting/outputting information as voice. 
     At first, the configuration of the vehicle navigation device according to the present embodiment will be described below with reference to FIG.  1 . 
     As shown in FIG. 1, a vehicle navigation device S is provided with an angular velocity sensor  1 , a travel distance sensor  2 , a GPS receiver  3 , a system controller  4 , an input unit  10  such as a remote control unit, a DVD-ROM drive  11 , a display unit  12 , an audio reproduction unit  17 , a voice recognition decoder  21 , a voice synthesizing unit  22  and a speaker  23 . The angular velocity sensor  1  detects an angular velocity of rotation of an vehicle so as to output angular velocity data and relative orientation data. The travel distance sensor  2  calculates a pulse number per rotation of wheels by counting a pulse number in a pulse signal of a predetermined period outputted according to rotation of the wheels so as to output travel distance data based on the pulse number per rotation. The GPS receiver  3  receives radio waves from GPS satellites so as to output GPS position measuring data and absolute orientation data indicating the advancing direction of the vehicle. The system controller  4  controls the whole navigation device based on the relative orientation data, the angular velocity data, the travel distance data, the GPS position measuring data and the absolute orientation data. The input unit  10  is used by a user or driver to input various data. The DVD-ROM drive  11  reads out and outputs various data such as map data including road data representing a number of traffic lanes, road widths and the like and data representing names of facilities and detailed information from a DVD-ROM (DVD-Read Only Memory) disk DK under the control by the system controller  4 . The display unit  12  displays the various display data under the control by the system controller  4 . The audio reproduction unit  17  reproduces and outputs various voice data under the control by the system controller  4 . The voice recognition decoder  21  recognizes contents of voices inputted from an external microphone  200 . The voice synthesizing unit  22  synthesizes voices based on the voice data outputted from the voice recognition decoder  21 . The speaker  23  outputs a synthesized voice signal outputted from the voice synthesizing unit  22  to be audible to the user or driver. 
     The system controller  4  includes an interface unit  5 , a CPU  6  as a computer, a ROM (Read Only Memory)  7  and a RAM  8 . The interface unit  5  performs an interface operation with the external sensors such as the GPS receiver  3 . The CPU  6  controls the whole system controller  4 . The ROM  7  stores a basic control program and the like for controlling the CPU  6 . The RAM  8  has a non-volatile memory, not shown, and stores various data, such as route data which has been set in advance by a user via the input unit  10  and data necessary for the processing in the CPU  6  in rewritable manner. The system controller  4  is connected with the input unit  10 , the DVD-ROM drive  11 , the display unit  12 , the audio reproduction unit  17  and the voice recognition decoder  21  via a bus line  9 . The control program for the information retrieving process, described later, executed by the CPU  6  is recorded beforehand in the DVD-ROM disk DK and is read out by the DVD-ROM drive  11  when it becomes necessary. 
     The display unit  12  includes a graphic controller  13 , a buffer memory  14 , a display control unit  15  and a display  16 . The graphic controller  13  controls the whole display unit  12  based on the control data transmitted from the CPU  6  via the bus line  9 . The buffer memory  14  may be composed of a memory such as a VRAM (Video RAM), and temporarily stores image information which can be displayed immediately. The display control unit  15  controls the display  16  composed of a liquid crystal display device, a CRT (Cathode Ray Tube) display device or the like based on image data outputted from the graphic controller  13 . 
     The audio reproduction unit  17  includes a D/A converter  18 , an amplifier  19  and a speaker  20 . The D/A converter  18  converts digital voice data transmitted from the DVD-ROM drive  11  or the RAM  8  via the bus line  9  into analog voice signal. The amplifier  19  amplifies the analog voice signal outputted from the D/A converter  18 . The speaker  20  converts the amplified analog voice signal into a voice so as to be outputted as an audible voice message from the navigation device. 
     The voice recognition decoder  21  has a voice recognition circuit for recognizing human voice. When a spoken word is inputted via the external microphone  200 , a candidate word which is closest to the spoken word is selected from an internal dictionary including information representing many words, and candidate word data representing the selected candidate word are outputted to the CPU  6  via the bus line  9 . Here, the internal dictionary may be stored in the DVD-ROM disk DK and read out by the DVD-ROM drive  11  according to needs. 
     In the vehicle navigation device S having the above-mentioned configuration, when a user inputs spot information or the like representing a destination spot via the external microphone  200  in the course of the processing procedure described later, the candidate word corresponding to the inputted word is retrieved by the voice recognition decoder  21 , and map data or the like stored in the DVD-ROM disk DK are read out by the system controller  4  based on the retrieved candidate word so as to be displayed on the display unit  12 . 
     Of course, a destination spot or the like may be inputted by the input unit  10 . Alternatively, however, such information may be inputted easily by voice input to dispense with an operation by buttons or the like on the input unit  10 . 
     In addition, in the vehicle navigation device S shown in FIG. 1, plural kinds of place names or facility names which are frequently set as a destination spot may be selected and registered, in advance, in the RAM  8  together with a number of times at which they are selected and adopted as the destination spot. 
     In addition, a plurality of place names or facility names which have been retrieved recently and set as a destination spot may be stored in the RAM  8 . 
     Next, with reference to FIGS. 2A and 2B, the description will be given of the data structures of request message data used for voice output from the speaker  23  and recognition words used for recognizing a voice inputted via the external microphone  200  (namely, a voice recognizing data corresponding to words to be inputted). 
     First, the description will be given of the data structure of the request message data with reference to FIG.  2 A. Here, a voice to be outputted from the speaker  23  in the present embodiment is a request message for requesting the user to input information by notifying the user of the content of the information to be inputted next, and  2 A shows the data structure of the request messages. The voice data to be outputted as the request message shown in FIG. 2A are recorded beforehand in the DVD-ROM disk DK, and they are read out by the DVD-ROM drive  11  according to need. The method of using respective messages will be described later by referring to flowcharts. 
     As shown in FIG. 2A, the request message SM of the present embodiment is classified into six hierarchies. 
     Voice data, which are used for outputting a message “Where are you going ?” just after the power-on of the vehicle navigation device S (more specifically, when an ignition switch of the vehicle is turned on), are stored in an area  50  in a zeroth hierarchy  30  which is the most significant hierarchy. 
     Next, voice data for outputting a message “One of prefecture, area code, registered address, genre and recently retrieved address, please. ”, which should be outputted via the speaker  23  after the request message SM “Where are you going?”, are stored in an area  51  of a first hierarchy  31 . 
     Next, voice data for outputting a message “Prefecture, please., which is associated with the word “prefecture” of the message in the area  51 , are stored in an area  52  in a second hierarchy  32 . 
     Next, voice data for outputting a message “City name, please.”, which is associated with the message in the area  52 , are stored in an area  53  in a third hierarchy  33 . 
     In addition, voice data for outputting a message “Address, please.”, which is associated with the message in the area  53 , are stored in an area  54  in a fourth hierarchy  34 . 
     Next, voice data for outputting a message “Area code, please.”, which is associated with the word “area code” of the message in the area  51 , are stored in an area  55  in the second hierarchy  32 . 
     In addition, voice data for outputting a message “Area code, please.”, which is associated with the message in the area  55 , are stored in an area  56  in the third hierarchy  33 . 
     Further, voice data for outputting a message “Remaining part of the phone number, please.”, which is associated with the message in the area  56 , are stored in an area  57  in the fourth hierarchy  34 . 
     Next, voice data for outputting a message “Address, please. ”, which is associated with the word “registered address” and the word “recently retrieved address” in the message in the area  51 , are stored in an area  58  in the second hierarchy  32 . 
     In addition, voice data for outputting a message “◯◯, χχ, ΔΔ, ♦♦, ⊚⊚, or the others?” as a message of the first segment, which is associated with the message in the area  58 , are stored in an area  59  in the third hierarchy  33 . 
     Further, voice data for outputting a message “, ▴▴, ∇∇, □□, or ⋄⋄?” as a message of the second segment, which is associated with the message in the area  58 , are stored in an area  60  in the third hierarchy  33 . 
     Here, the above-mentioned “◯◯”, “χχ”, “ΔΔ”, “♦♦”, “⊚⊚”, “”, ▴▴”, “∇∇”, “□□”, and “⋄⋄”, represent concrete registered addresses which are used frequently or concrete addresses which are different from each other and have been retrieved recently. 
     Next, voice data for outputting a message “Station, highway, amusement park, golf course, ski area or the others?” as a message of the first segment, which is associated with the word “genre” in the message in the area  51 , are stored in an area  61  in the second hierarchy  32 . 
     In addition, voice data for outputting a message “accommodation facility, hospital, educational facility, park, campsite or the others?” as a message of the second segment, which is associated with the word “genre”, are stored in an area  62  in the second hierarchy  32 . 
     Further, voice data for outputting a message “Resort facility, public office, ferry terminal or airport?” as a message of the third segment, which is associated with to the word “genre”, are stored in an area  63  in the second hierarchy  32 . 
     Next, information suggesting the access to the area  52  in order to output the message (“Prefecture, please.”) in the area  52  according to the message in the area  61 ,  62  or  63 , are stored in an area  64  in the third hierarchy  33 . 
     Next, information suggesting the access to the area  53  in order to output the message (“City name, please.”) in the area  53  according to the information in the area  64 , are stored in an area  65  in the fourth hierarchy  34 . 
     Finally, information suggesting the access to the area  54  in order to output the message (“Address, please.”) in the area  54  according to the information in the area  65 , are stored in an area  66  in the fifth hierarchy  35 . 
     Here, as is clear from the data structure of the request messages SM, as for the request messages SM in the hierarchies not higher than the second hierarchy  32 , request message SM in the upper hierarchy is associated with one request message SM in the lower hierarchy. Therefore, the request message SM in the lower hierarchy is not associated with plural request messages SM in the upper hierarchies. 
     Next, the description will be given of the data structure of the recognition words W with reference to FIG.  2 B. Here, the recognition words in the present embodiment are words used to recognize the voice that a user inputs in response to the request messages SM. FIG. 2B shows a data structure of the recognition words W. The voice data corresponding to the recognition words shown in FIG. 2B are stored beforehand in the DVD-ROM disk DK as the internal dictionary and are read out by the DVD-ROM drive  11  according to need. The method of using the following recognition words will be described later together with flowcharts. 
     As shown in FIG. 2B, the recognition words W of the present embodiment are classified into five hierarchies so as to correspond to the respective hierarchies of the above request messages SM. 
     A dictionary, which shows the words “prefectures”, “area code”, “registered addresses”, “genre” and “recently retrieved address” in order to recognize a voice inputted in response to the message in the area  51  in the request messages SM, are stored in an area  70  in a first hierarchy  40  which is the most significant hierarchy. 
     Next, a dictionary, which shows real names of the prefectures (“Tokyo-To”, “Kanagawa-Ken” and the like) associated with the word “prefecture” in the message in the area  70  and used for recognizing a voice inputted in response to the word “Prefecture, please.” in the area  52  in the request messages SM, is stored in an area  71  in a second hierarchy  41 . 
     In addition, a dictionary, which shows real names of the city (“Shinagawa-Ku”, “Yokohama-Shi” and the like) associated with the word in the area  71  and used for recognizing a voice inputted in response to the word (“City name, please.”) in the area  53  in the request message SM, is stored in an area  72  in a third hierarchy  42 . 
     Next, a dictionary, which shows concrete addresses associated with the word in the area  72  and used for recognizing a voice inputted in response to the phrase (“Address, please.”) in the area  54  in the request message SM, is stored in an area  73  in a fourth hierarchy  43 . 
     In addition, a dictionary, which shows the area codes (“03”, “045” and the like) associated with the word “area codes” in the message in the area  70  and used for recognizing a voice inputted in response to the phrase (“Area code, please.”) in the area  55  in the request message SM, is stored in an area  74  in the second hierarchy  41 . 
     Next, a dictionary, which shows the area codes (“111”, “987” and the like) associated with the word in the area  74  and used for recognizing a voice inputted In response to the phrase (“Area code, please.”) in the area  56  in the request message SM, is stored in an area  75  in the third hierarchy  42 . 
     Further, a dictionary, which shows telephone numbers (“2345”, “9876” and the like) associated with the word in the area  75  and used for recognizing a voice inputted in response to the phrase (“Remaining part of the phone number, please.”) in the area  57  in the request message SM, is stored in an area  76  in the fourth hierarchy  43 . 
     In addition, a dictionary, which shows concrete addresses associated with the word “registered addresses” and the word “recently retrieved addresses” in the messages in the area  70  and used for recognizing a voice inputted in response to the phrase (“Address, please.”) in the area  58 , in the request message SM, is stored in an area  77  in the second hierarchy  41 . 
     Next, a dictionary, which shows the concrete words “◯◯”, “χχ”, “ΔΔ”, “♦♦”, “⊚⊚”, and “the others” associated with the word in the area  77  and used for recognizing a voice inputted in response to the phrase (“◯◯, χχ, ΔΔ, ♦♦, ⊚⊚ or the others?”) in the area  59  in the request message SM, is stored in an area  78  in the third hierarchy  42 . 
     In addition, a dictionary, which shows the concrete words “”, “▴▴”, “∇∇”, “□□”, and “⋄⋄” associated with the word in the area  77  and used for recognizing a voice inputted in response to the phrase (“, ▴▴, ∇∇, □□, or ⋄⋄?”) in the area  60  in the request message SM, is stored in an area  79  in the third hierarchy  42 . 
     Next, a dictionary, which shows the words “station”, “highway facility”, “amusement park”, “golf course”, “ski area” and “the others” associated with the word “genre” in the message in the area  70  and used for recognizing a voice inputted in response to the phrase (“Station, highway facility, amusement park, golf course, ski area or the others?”) in the area  61  in the request message SM, is stored in the area  79  in the second hierarchy  41 . 
     In addition, a dictionary, which shows the words “accommodation facility”, “hospital”, “educational facility”, “park”, “campsite and “the others” associated with the word “genre” in the message in the area  70  and used for recognizing a voice inputted in response to the phrase (“Accommodation facility, hospital, educational facility, park, campsite or the others?”) in the area  62  in the request message SM, is stored in an area  80  in the second hierarchy  41 . 
     Further, information suggesting the access to the area  71  associated with the word in the area  79 ,  80  or  81  and used to wait for voice input using the words in the area  71  (real names of the prefectures) in response to the information in the area  64  in the request message SM is stored in an area  82  in the third hierarchy  42 . 
     Next, information suggesting the access to the area  72  associated with the word in the area  82  and used to wait for voice input by using the words (real names of the prefectures) in the area  72  in response to the information in the area  65  in the request message SM is stored in an area  83  in the fourth hierarchy  43 . 
     Next, information suggesting the access to the area  73  associated with the word in the area  83  and used to wait for voice input by using the words (real addresses) in the area  73  in response to the information in the area  66  in the request message SM is stored in an area  84  in the fifth hierarchy  44 . 
     Here, as is clear from the data structure of the recognition words W, as for the recognition words W in the hierarchies not higher than the second hierarchy  41 , one recognition word in the upper hierarchy is associated with one recognition word W in the lower hierarchy. Therefore, the recognition word W in the lower hierarchy is not associated with plural recognition words W in the upper hierarchy. Therefore, it is not necessary to wait for voice input using all the recognition words W, and thus it may be enough to wait for voice input using the words in the lower hierarchies associated with the selected words in the upper hierarchies. 
     Here, even if a voice “Shinagawa-Ku”, for example, is inputted in response to the message “Prefecture, please.” in the request message SM, it is preferred to recognize this voice input. In order to achieve this, when waiting for voice input after the request message SM in the first hierarchy  31  is outputted, not only the recognition word W in the first hierarchy  40  corresponding to the outputted request message SM but also the respective recognition words in the second, third, fourth and fifth hierarchies  41 ,  42 ,  43  and  44  associated with the recognition word W in the first hierarchy  40  may be referred to. 
     Similarly to this, when waiting for voice input after the request message SM in the second hierarchy  32  associated with the request message SM in the first hierarchy, not only the recognition word W in the second hierarchy  41  corresponding to the outputted request message SM but also the respective recognition words in the third, fourth and fifth hierarchies  42 ,  43  and  44  associated with the recognition word W in the second hierarchy  41  may be referred to. 
     Further, when waiting for voice input after the request message SM in the third hierarchy  33  associated with the request message SM in the second hierarchy  32  selected previously is outputted, not only the recognition word W in the third hierarchy  42  corresponding to the outputted request message SM but also the respective recognition words W in the fourth and fifth hierarchies  43  and  44  associated with and according to the recognition word W in the third hierarchy  42  may be referred to. 
     Next, the information retrieval process according to the present invention using the request messages SM and the recognition words W having the data structure shown in FIGS. 2A and 2B will be described with reference to the flowcharts shown in FIGS. 3 through 12. Here, in the respective flowcharts, “first request voice is outputted”, for example, means that a voice corresponding to the message in the first hierarchy  31  in the request messages SM is outputted. Moreover, the operations shown in the respective flowcharts are performed by CPU  6  by reading out and executing the control programs stored in the DVD-ROM disk DK. 
     As shown in FIG. 3, in the information retrieving process of the present embodiment, at first, when an ignition key or the like of a vehicle is turned on, a judgment is made as to whether or not the power source of the vehicle navigation device S is turned on (step S 1 ). Then, if the power source is not turned on (step S 1 : NO), the turning-on of the power source is waited, and if the power source is turned on (step S 1 ; YES), the initial setting including initialization of the RAM  8  and the like is executed (step S 2 ). 
     Thereafter, if an information retrieval mode is started by an input operation via the input unit  10  or the like (step S 3 ), the zeroth request voice “Where are you going?” is outputted through the speaker  23  (step S 4 ). 
     Next, a judgment is made as to whether or not voice input responding to the zeroth request voice (hereinafter, a voice input in response to respective request voices is simply referred to as “response”) is executed (step S 5 ). If a response is made (step S 5 ; YES), a content of the response is recognized by the voice recognition decoder  21  (step S 8 ), and then the hierarchy of the recognition word W corresponding to the content of the response is judged (step S 9 ). The detail of the step S 9  will be described later. 
     On the other hand, if no response is made for a predetermined time set previously in the judgment at step S 5  (step S 5 ; NO), the first request voice “One of prefecture, area code, registered address, recently retrieved address or genre, please.” is outputted through the speaker  23  (step S 6 ). A judgment is then made as to whether or not a response is made (step S 7 ). If no response is made for a predetermined time (step S 7 ; NO), it is regarded that the driver does not want to execute the information retrieval process so that the information retrieval process is ended. If a response is made at step S 7  (step S 7 ; YES), the sequence goes to step S 8  to recognize the content of the response, and goes to step S 9 . 
     Next, the hierarchy judgment at the step S 9  will be described in detail with reference to FIG.  4 . 
     In the hierarchy judging process at the step S 9 , at first, a judgment is made as to whether or not the response at the step S 5  is the word included in the first hierarchy  40  in the recognition words W (step S 10 ). If the response is the word included in the first hierarchy  40  (step S 10 ; YES), the sequence goes to step S 20  described later. 
     On the contrary, if the response is not the word included in the first hierarchy  40  (Step S 10 ; NO) in the judgment at step S 11 , a judgment is made as to whether or not the response at step S 5  is a real name representing a prefecture (for example, Tokyo”, “Kanagawa” or the like) (step S 11 ). 
     If the response is a real name representing the prefecture (step S 11 ; YES), the sequence goes to step S 32  described later. If the response is not a real name representing a prefecture (step S 11 ; NO), a judgment is made as to whether or not the response at step S 5  is a real name representing a city name (for example, “Shinagawa-Ku”, “Yokohama-Shi” or the like) (step S 12 ). 
     If the response is a real name representing a city (step S 12 ; YES), the sequence goes to step S 73 . If the response is not a real name representing a city (step S 12 ; NO), a judgment is made as to whether or not the response at S 5  is a real name representing the facilities or places (for example, “Tokyo Dome”, “Yokohama Stadium” or the like) (step S 13 ). 
     If the response is a real name representing the facilities or places (step S 13 ; YES), in order to display a map where the facility or place exists, a map including the facility or place is retrieved, and map data corresponding to the retrieved map are read out from the DVD-ROM disk DK and displayed (step S 17 ). Then, the process is ended. 
     On the other hand, if the response at S 5  is neither a facility name nor a place name in the judgment at step S 13  (step S 13 : NO), a judgment is made as to whether or not the response at S 5  is a number (numeral) representing an area code of a telephone number (for example, “03”, “045” or the like) (step S 14 ). 
     If the response is a number representing an area code (step S 14 , YES), the sequence goes to step S 51 . If the response is not a number representing an area code (step S 14 ; NO), a judgment is made as to whether or not the response at S 5  is a real name representing the destination spot registered previously (for example “China town”, “Roppongi” or the like) (step S 15 ). 
     If the response is the registered destination spot name (step S 15 , YES), the sequence goes to step S 17  so that a map is retrieved and displayed. If the response is not the registered destination spot name (step S 15 ; NO), a judgment is made as to whether or not the response at S 5  is a real name representing the place which has been recently retrieved and stored (step S 16 ). 
     If the response is the recently retrieved place (step S 16 ; YES), the sequence goes to step S 17  so that a map is retrieved and displayed. If the response is not the retrieved place (step S 16 ; NO), the sequence goes to step S 6  so that the aforementioned process is repeated. 
     Next, the description will be given of a process executed when the judgement in step S 10  results in that the response at step S 5  is a content included in the first hierarchy  40 , with reference to FIG.  5 . 
     As shown in FIG. 5, if the response at S 5  is a content included in the first hierarchy  40  in the judgment at step S 10  (step S 10 . YES), a judgment is made as to whether or not the response is the word “Prefecture” (step S 20 ). If the response is the word “Prefecture” (step S 20 ; YES), the sequence goes to S 25  described later. 
     On the other hand, if the response is not the word “Prefecture” (step S 20 ; NO), a judgment is made as to whether or not the response at S 5  is the word “Area code” (step S 21 ). If the response is the word “Area code” (step S 21 ; YES), the sequence goes to step S 47  described later. 
     If the response is not the word “Area code” (step S 21 ; NO), a judgment is made as to whether or not the response at S 5  is the word “Registered address” (step S 22 ). If the response is the word “Registered address” (step S 22 ; YES), the sequence goes to S 90  described later. 
     If the response is not the word “Registered address” (step S 22 ; NO), a judgment is made as to whether or not the response at S 5  is the word “Genre” (step S 23 ). If the response is the word “Genre” (step S 23 ; YES), the sequence goes to step S 59  described later. 
     If the response is not the word “Genre” (step S 23 ; NO), a judgment is made that the response at S 5  is “Recently retrieved place” (step S 24 ), and the sequence goes to step S 100  described later. 
     Next, the description will be given of a process in the case where the response at step S 5  is the word “Prefecture” in the above step S 20  with reference to FIG.  6 . 
     As shown in FIG. 6, if the response at step S 5  is the word “Prefecture” in the judgment at step S 20  (step S 20 ; YES), a parameter i, which represents a number of times that the request message “Prefecture, please.” is outputted, is initialized (step S 25 ), and the second request voice “Prefecture, please.” is outputted through the speaker  23  (step S 26 ). 
     Then, a judgment is made as to whether or not a response is made (step S 27 ). If no response is made for a predetermined time (step S 27 ; NO), the parameter i is incremented only by one (step S 28 ), and a judgment is made as to whether or not the value of the increased parameter i is “2” (step S 29 ). If the value is “2” (step S 29 ; YES), this means that a response is not made even after the request message “Prefecture, please.” is outputted twice. Therefore, a judgment is made that the driver does not want to retrieve information so that the process is ended. 
     On the other hand, if the parameter i is not “2” in the judgment at step S 29 , the sequence returns to step S 26  in order to again output the request message “Prefecture, please.”. 
     If a response is made to the request message “Prefecture, please.” in the judgment at step S 27  (step S 27 ; YES), the response is recognized by the voice recognition decoder  21  (step S 30 ), and the hierarchy of the recognized content is determined (step S 31 ). The detailed process at step S 31  will be described later. 
     When the hierarchy of the response content recognized at step S 30  is determined, the third request voice “City name, please.” is outputted through the speaker  23  (step S 32 ). 
     Then, a judgment is made as to whether or not a response is made (step S 33 ). If no response is made for a predetermined time (step S 33 ; NO), a map including the city represented by the word is retrieved based on the word representing the city name in the content recognized at step S 30 , and map data corresponding to the retrieved map are read out from the DVD-ROM disk DK and displayed (step S 34 ). Then, the process is ended. 
     On the other hand, if a response is made to the request message “City name, please.” in the judgment at step S 33  (step S 33 ; YES), the response is recognized by the voice recognition decoder  21  (step S 35 ), and the hierarchy of the recognized content is determined (step S 36 ). The detailed process at step S 36  will also be described later. 
     If the hierarchy of the response content recognized at step. S 35  is determined, the fourth request voice “Address, please.” is outputted through the speaker  23  (step S 37 ). 
     Then, a judgment is made as to whether or not a response is made (step S 38 ). If no response is made for a predetermined time (step S 38 ; NO), a map including the city represented by the word is retrieved based on the word representing the city in the content recognized at step S 35 , and map data corresponding to the retrieved map are read out from the DVD-ROM disk DK and displayed (step S 39 ). Then, the process is ended. 
     On the other hand, if a response is made to the request message “Address, please.” in the judgment at step S 38  (step S 38 ; YES), the response is recognized by the voice recognition decoder  21  (step S 40 ), and the sequence goes to step S 17  so that the map data are retrieved and displayed based on the recognized content. 
     Next, the description will be given of the content of the hierarchy determination process at the steps S 31  and S 36  with reference to FIGS. 7A and 7B. 
     At first, the description will be given as to the content of the hierarchy determination process at step S 31  with reference to FIG.  7 A. First, a judgment is made as to whether or not the response at step S 27  is a real name representing a prefecture (step S 42 ). If the response is a real name representing the prefecture (step S 42 ; YES), the sequence goes to step S 32  so that the aforementioned operation is repeated. On the other hand, if the response is not a real name presenting a prefecture (step S 42 : NO), a judgment is made as to whether or not the response at step S 27  is a real name representing a city name (step S 43 ). If the response is a real name representing a city name (step S 43 ; YES), the sequence goes to step S 37  so that the aforementioned operation is repeated. On the other hand, if the response is not a real name representing a city name (step S 43 ; NO), a judgment is made as to whether or not the response at step S 27  is a real name representing a facility name or a place name (step S 44 ). If the response is a real name representing a facility name or a place name (step S 44 ; YES), the sequence goes to step S 17  so that map data are retrieved based on the facility name or the place name, and a corresponding map is displayed. If the response is not a real name representing a facility name or a place name (step S 44 ; NO), the sequence goes to step S 25  so that the aforementioned operation is repeated. 
     Next, the description will be given as to the content of the hierarchy determination process at step S 36  with reference to FIG.  7 B. At first, a judgment is made as to whether or not the response at step S 33  is a real name representing a city name (step S 45 ). If the response is a real name representing a city name (step S 45 ; YES), the sequence goes to step S 37  so that the aforementioned operation is repeated. If the response is not a real name representing a city name (step S 45 ; NO), a judgment is made as to whether or not the response at step S 33  is a real name representing a facility name or a place name (step S 46 ). If the response is a real name representing a facility name or a place name (step S 46 ; YES), the sequence goes to step S 17  so that map data are retrieved based on the facility name or the place name, and a corresponding map is displayed. On the other hand, if the response is not a real name representing a facility name or a place name (step S 46 ; NO), the sequence goes to step S 25  so that the aforementioned operation is repeated. 
     Next, the description will be given of the process executed when it is determined in step S 21  that the response at step S 5  is the word “Area code”, with reference to FIG.  8 . 
     As shown in FIG. 8, if the response at step S 5  is the word “Area code” in the judgment at step S 21  (step S 21 ; YES), the first request voice “Area code, please.” is outputted through the speaker  23  (step S 47 ). 
     Then, a judgment is made as to whether or not a response is made (step S 48 ). If no response is made for a predetermined time (step S 48 ; NO), the sequence again returns to step S 47  so that the first request voice “Area code, please.” is outputted. If a response is made (step S 48 ; YES), the response is recognized by the voice recognition decoder  21  (step S 49 ), and a judgment is made as to whether or not the recognized content is an area code (step S 50 ). 
     If the content is not an area code (step S 50 ; NO), the sequence again returns to step S 47  so that the second request voice “Area code, please.” is outputted. If the content is an area code (step S 50 ; YES), the third request voice “City telephone number, please.” is outputted through the speaker  23  (step S 51 ). 
     Then, a judgment is made as to whether or not a response is made (step S 52 ). If no response is made for a predetermined time (step S 52 ; NO), the sequence again returns to step S 51  so that the third request voice “City telephone number, please.” is outputted. When a response is made (step S 52 ; YES), the response is recognized by the voice recognition decoder  21  (step S 53 ), and a judgment is made as to whether or not the recognized content is a city telephone number (step S 54 ). 
     If the content is not a city telephone number (step S 54 ; NO), the sequence again returns to step S 51  so that the third request voice “City telephone number, please.” is outputted. If the content is a city telephone number (step S 54 ; YES), the fourth request voice “Remaining part of the phone number, please.” is outputted through the speaker  23  (step S 55 ). 
     Then a judgment is made as to whether or not a response is made (step S 56 ). If no response is made for a predetermined time (step S 56 : NO), the sequence again returns to step S 55  so that the fourth request voice “Remaining part of the phone number, please.” is outputted. If a response is made (step S 56 ; YES), the response is recognized by the voice recognition decoder  21  (step S 57 ), and a judgment is made as to whether or not the recognized content is the remaining part of the number (step S 58 ). 
     If the content is not the remaining part of the number (step S 58 ; NO), the sequence again returns to step S 55  so that the fourth request voice “Remaining part of the number, please.” is outputted. If the content is the remaining part of the number (step S 58 ; YES), the sequence returns to step S 17  so that map data are retrieved based on the inputted telephone number (steps S 48 , S 52  and S 56 ) and a corresponding map is displayed. 
     Next, the description will be given of the process executed when it is determined at the step S 23  that the response at step S 5  is the word “genre name”, with reference to FIG.  9 . 
     As shown in FIG. 9, if the response at step S 5  is the word “Genre” in the judgment at step S 23  (step S 23 ; YES), the first segment of the second request voice corresponding to the genre name “Station, amusement park, freeway, golf course, ski area or the others ?” is outputted through the speaker  23  (step S 59 ). 
     Then, a judgment is made as to whether or not a response is made (step S 60 ). If no response is made for a predetermined time (step S 60 ; NO), the sequence goes to step S 78  described later. 
     If a response is made in the judgment at step S 60  (step S 60 ; YES), the response is recognized by the voice recognition decoder  21  (step S 61 ), and a judgment is made as to whether or not the recognized content is the word “The others” (step S 62 ). If the content is the word “The others” (step S 62 ; YES), the second segment of the second request voice corresponding to the genre name “Accommodation facility, hospital, educational facility, park, campsite, or the others ?” is outputted through the speaker  23  (step S 78 ). 
     Then, a judgment is made as to whether or not a response is made (step S 79 ). If no response is made for a predetermined time (step S 79 ; NO), the sequence goes to step S 82  described later. 
     If a response is made in the judgment at step S 79  (step S 79 : YES), the response is recognized by the voice recognition decoder  21  (step S 80 ), and a judgment is made as to whether or not the recognized content is the word “The others” (step S 81 ). If the content is the word “The others” (step S 81 ; YES), the third segment of the second request voice corresponding to the genre name “Resort facility, public office, ferry terminal or airport?” is outputted through the speaker  23  (step S 82 ). 
     Then, a judgment is made as to whether or not a response is made (step S 83 ). If no response is made for a predetermined time (step S 83 ; NO), the sequence returns to the step S 59  so that the aforementioned operation is repeated. 
     If a response is made in the judgment at step S 83  (step S 83 ; YES), the response is recognized by the voice recognition decoder  21  (step S 84 ), and the sequence goes to step S 63 . 
     If the recognized content is not the word “The others” in the judgment at step S 62  or S 81  (step S 62 ; N 0  or S 81 ; NO), the third request voice “Prefecture, please.” is outputted through the speaker  23  (step S 63 ). 
     Then, a judgment is made as to whether or not a response is made (step S 64 ). If no response is made for a predetermined time (step S 64 ; NO), the sequence goes to step S 67 . If a response is made (step S 64 ; YES), the response is recognized by the voice recognition decoder  21  (step S 65 ), and the hierarchy of the recognized content is determined (step S 66 ). The detailed process at step S 66  will be described later. 
     If the hierarchy of the response content recognized at step S 66  is determined, the fourth request voice “City name, please.” is outputted through the speaker  23  (step S 67 ). 
     Then, a judgment is made as to whether or not a response is made (step S 68 ). If a response is made (step S 68 ; YES), the response is recognized by the voice recognition decoder  21  (step S 71 ), and the hierarchy of the recognized content is determined (step S 72 ). The detailed process at step S 72  will be described later. 
     On the other hand, if a response is not made for a predetermined time in the judgment at step S 68  (step S 68 ; NO), a judgment is made as to whether or not that the response is recognized at the step S 65  (step S 69 ). If the response is recognized (step S 69 ; YES), a map including the city represented by the word is retrieved based on the recognized word representing the city name, and map data corresponding to the retrieved map are read out from the DVD-ROM disk DK so as to be displayed (step S 70 ). Then, the process is ended. 
     On the other hand, if the response is not recognized in the judgment at step S 69  or the hierarchy determination is completed at step S 72 , the fifth request voice “Address or facility name, please.” is outputted through the speaker  23  (step S 73 ). 
     Then, a judgment is made as to whether or not a response is made (step S 74 ). If no response is made for a predetermined time (step S 74 ; NO), a judgment is made as to whether or not the response is recognized at the step S 71  (step S 75 ). If the response is recognized (step S 75 ; YES), a map including the city represented by the word is retrieved based on the recognized word representing the city name, and map data corresponding to the retrieved map are read out from the DVD-ROM disk DK and displayed (step S 70 ). Then, the process is ended. 
     On the other hand, if the response is not recognized in the judgment at step S 75  (step S 75 ; NO), a judgment is made as to whether or not the response is recognized at the step S 65  (step S 76 ). If the response is recognized (step S 76 ; YES), a map including the city represented by the recognized word is retrieved based on the word representing the city name, and map data corresponding to the retrieved map are read out from the DVD-ROM disk DK and displayed (step S 70 ). Then, the process is ended. 
     If the response is not recognized in the judgment at step S 76  (step S 76 ; NO), the sequence returns to the step S 73  so that the aforementioned operation is repeated. 
     Further, if a response is made in the judgment at step S 74  (step S 74 ; YES), the response is recognized by the voice recognition decoder  21  (step S 77 ), and the sequence returns to step S 17  so that map data are retrieved based on the inputted address or facility name and a corresponding map is displayed. 
     Next, the content of the hierarchy determination process at the steps S 66  and S 72  will be described with reference to FIGS. 10A and 10B. 
     At first, the description will be given as to the content of the hierarchy determination process at step S 66  with reference to FIG.  10 A. First, a judgment is made as to whether or not the response at step S 64  is a real name representing a city, town or village name (step S 85 ). If the response is a real name representing a city name (step S 85 ; YES), the sequence goes to step S 67  so that the aforementioned operation is repeated. On the other hand, if the response is not a real name representing a city name (step S 85 ; NO), a judgment is made as to whether or not the response at step S 64  is a real name representing a city name (step S 86 ). If the response is a real name representing a city name (step S 86 ; YES), the sequence goes to step S 73  so that the aforementioned operation is repeated. If the response is not a real name representing a city name (step S 86 ; NO), a judgment is made as to whether or not the response at step S 64  is a real name representing a facility name or a place name (step S 87 ). If the response is a real name representing a facility name or a place name (step S 87 ; YES), the sequence goes to step S 17  so that map data are retrieved based on the facility or place name and a corresponding map is displayed. If the response is not a real name representing the a facility name or a place name (step S 87 ; NO), the sequence goes to step S 63  so that the aforementioned operation is repeated. 
     Next, the description will be given as to the content of the hierarchy judging process at step S 72  with reference to FIG.  10 B. At first, a judgment is made as to whether or not the response at step S 68  is a real name representing a city name (step S 88 ). If the response is a real name representing a city name (step S 88 ; YES), the sequence goes to step S 73  so that the aforementioned operation is repeated. On the other hand, if the response is not a real name representing a city name (step S 88 : NO), a judgment is made as to whether or not the response at step S 68  is a real name representing a facility name or a place name (step S 89 ). If the response is a real name representing a facility name or a place name (step S 89 ; YES), the sequence goes to step S 17  so that map data are retrieved based on the facility or place name and a corresponding map is displayed. If the response is not a real name representing a facility name or a place names (step S 89 ; NO), the sequence goes to step S 73  so that the aforementioned operation is repeated. 
     Next, the process executed when it is determined at the step S 22  that the response at step S 5  is a word “Registered address” will be described with reference to FIG.  11 . 
     As shown in FIG. 11, if the response at step S 5  is the word “Registered address” (step S 22 ; YES), the second request voice “Address, please.” is outputted through the speaker  23  (step S 90 ). 
     Then, a judgment is made as to whether or not a response is made (step S 91 ). If no response is made for a predetermined time (step S 91 ; NO), the sequence goes to step S 93  described later. On the other hand, if a response is made (step S 91 : YES), the response is recognized by the voice recognition decoder  21  (step S 92 ), and the hierarchy determination process similar to step S 31  is executed. 
     On the other hand, if no response is made for a predetermined time in the judgment at step S 91  (step S 91 ; NO), the first segment of the third request voice corresponding to the registered address “◯◯, χχ, ΔΔ, ♦♦, ⊚⊚, or the others ?” is outputted through the speaker  23  (step S 93 ). Then, a judgment is made as to whether or not a response is made (step S 94 ). If no response is made for a predetermined time (step S 94 ; NO), the sequence goes to step S 97  described later. 
     If a response is made in the judgment at step S 94  (step S 94 ; YES), the response is recognized by the voice recognition decoder  21  (step S 95 ), and a judgment is made as to whether or not the recognized content is the word “The others” (step S 96 ). 
     If the content is not the word “The others” (step S 96 ; NO), the sequence goes to step S 31 . If the content is the word “The others” (step S 96 ; YES), the second segment of the third request voice corresponding to the registered address “, ▴▴, ∇∇, □□, or ⋄⋄?” is outputted through the speaker  23  (step S 97 ). 
     Then, a judgment is made as to whether or not a response is made (step S 98 ). If no response is made for a predetermined time (step S 98 ; NO), the sequence returns to the step S 90  so that the aforementioned operation is repeated. 
     If a response is made in the judgment at step S 98  (step S 98 : YES), the response is recognized by the voice recognition decoder  21  (step S 99 ), and the hierarchy determination at step S 31  is executed. 
     Next, the process to be executed after the process at the step S 24  will be described with reference to FIG.  12 . 
     As shown in FIG. 12, if a judgment is made at step S 24  that the response at step S 5  is the word “Recently retrieved address”, the second request voice “Address, please. ”is outputted through the speaker  23  (step S 100 ). 
     Then, a judgment is made as to whether or not a response is made (step S 101 ). If no response is made for a predetermined time (step S 101 ; NO), the sequence goes to step S 103  described later. On the other hand, if a response is made (step S 101 ; YES), the response is recognized by the voice recognition decoder  21  (step S 102 ), and the hierarchy determination process similar to that at the step S 31  is executed. 
     If no response is made for a predetermined time in the judgment at step S 101  (step S 101 ; NO), the first segment of the third request voice corresponding to the recently retrieved place “◯◯, χχ, ΔΔ, ♦♦, ⊚⊚, or the others?” is outputted through the speaker  23  (step S 103 ). 
     Then, a judgment is made as to whether or not a response is made (step S 104 ). If no response is made for a predetermined time (S 104 ; NO), the sequence goes to step S 107  described later. 
     If a response is made in the judgment at step S 104  (step S 104 ; YES), the response is recognized by the voice recognition decoder  21  (step S 105 ), and a judgment is made as to whether or not the recognized content is the word “The others” (step S 106 ). If the content is not the word “The others” (step S 106 ; NO), the sequence goes to step S 31 . If the content is the word “The others” (step S 106 ; YES), the second segment of the third request voice corresponding to the recently retrieved place“ , ▴▴, ∇∇, □□ or ⋄⋄?” is outputted through the speaker  23  (step S 107 ). 
     Then, a judgment is made as to whether or not a response is made (step S 108 ). If no response is made for a predetermined time (step S 108 ; NO), the sequence returns to the step S 100  so that the aforementioned operation is repeated. 
     If a response is made in the judgment at step S 108  (step S 108 ; YES), the response is recognized by the voice recognition decoder  21  (step S 109 ), and the aforementioned hierarchy judgment at step S 31  is executed. 
     As described above, according to the information retrieval process executed in the vehicle navigation device S of the invention, since a hierarchy of inputted voice information is judged and a destination spot is retrieved based on the judged hierarchy, it is possible to retrieve the destination spot in the order of hierarchies, or to retrieve the destination spot while skipping one or plural hierarchies and shifting from a hierarchy to a hierarchy, for example. 
     In addition, the respective recognition words W belonging to the n+1 (n: natural number in a range of 2 to 4) hierarchy are associated with only one of the recognition words W included in the n hierarchy. Namely, if the recognition words W in the upper hierarchies are different, the candidate information of  1  recognition words W in the lower hierarchy are different. Therefore, the hierarchical structure can be constituted to include plural types of recognition words W. 
     Further, if a judgment is made that the hierarchy of voice information inputted in response to the request message SM in the p-th hierarchy (p: natural numbers not more than 5) is the p-th hierarchy, the request message SM for inputting voice information belonging to the (p+1)-th hierarchy is outputted through the speaker  23 . If a judgment is made that the hierarchy of the voice information inputted in response to the request message SM in the p-th hierarchy is (p+q)-th (p+q: natural numbers not more than 4) hierarchy, the request message SM for inputting voice information belonging to (p+q+1)-th hierarchy is outputted through the speaker  23 . Therefore, a destination spot can be retrieved in the order of the hierarchy structure, or a destination spot can be retrieved while skipping one or plural hierarchies. 
     Furthermore, if voice information belonging to a hierarchy represented by an outputted request message SM is not inputted, a request message SM for requesting the input of voice information in one-level lower hierarchy with respect to the former request message SM is outputted. As a result, a desired destination spot can be retrieved quickly. 
     In addition, since recognition word W belonging to the third hierarchy  42  corresponding to the registered address or recently retrieved address is divided into plural segments to be outputted, a time required for outputting one request message SM can be shortened. Moreover, desired voice information can be inputted without the need to output a request message SM corresponding to all the recognition words W belonging to one hierarchy. 
     Further, since information can be inputted and outputted by a voice, the convenience of the information retrieval process is further improved. 
     In addition, in the recognition words W shown in FIG. 2, if one recognition word W in the first hierarchy  40  is selected, for example, voice input is waited for the recognition words belonging to the hierarchies not higher than the second hierarchy  41  by using only the recognition words W associated with the recognition words W in the first hierarchy  40 . As a result, a number of the recognition words W for waiting can be reduced, and erroneous recognition of responses can be avoided. 
     Further, as for the recognition words W in each hierarchy, their frequency of use may be recorded, and recognition words W, which are used more frequently, may have priority in use for waiting, or recognition words W, whose content is closer to the contents of recognition words W currently used for waiting, may have priority in use for the recognition of responses. As a result, a number of the recognition words W used in recognizing the responses may be further reduced, and erroneous recognition of responses can be further reduced. 
     Here, the above embodiment described the case where the present invention is applied to the vehicle navigation device S, but the present invention can also be applied to an aircraft navigation device and a vessel navigation device.