DESCRIPTION Title of the Invention CHARGING SYSTEM AND METHOD FOR CONTROLLING CHARGING SYSTEM Technical Field [0001] The present invention relates to a charging system and a method for controlling charging system for charging an electric vehicle, which travels with charged electric power, such as an electric car or a plug-in hybrid car. Background Art [0002] In recent years, car sharing where a plurality of households share one car (or a plurality of cars) has started to become widespread. Currently, the majority of vehicles used for car sharing travel with gasoline. However, in many cases, since the distance which the vehicle travels in one use is short (for example, for shopping or picking up and dropping off), there is a high possibility that an electric vehicle such as an electric car or a plug-in hybrid car will be used in the future. [0003] Further, if car sharing is introduced for a housing complex, rather than providing one or more parking facilities for each household, car sharing is performed and parking facilities for the entire housing complex are reduced, providing the advantage that the land area can be effectively used. [0004] For example, at the beginning of car sharing introduction, such a system can be considered in which a parking area corresponding to one vehicle is prepared for each household and car sharing is used when a second or further vehicles are required. A resident uses their own vehicle for commuting on weekdays or trips on days off, and makes use of car sharing when their own vehicle is being used outside or is inconvenient to use. [0005] If the frequency of use of car sharing amongst residents of the housing complex increases and the number of households which do not require their own vehicles is increased, it will be possible to reduce individual parking areas, and to increase the value of the housing complex by installing the likes of solar power generation devices or by renting out a parking area to a third party for example. [0006] In this way since there are households owning their own vehicle among the households which participate in the car sharing, both individually owned vehicles and shared vehicles are present together at the housing complex. Further, the individually owned vehicle can be either an electric vehicle or a non-electric vehicle (such as gasoline cars and diesel cars). Thus, there is a possibility that at least three types of vehicles, including electric vehicles shared by a plurality of residents, electric vehicles individually owned by a resident and non-electric vehicles individually owned by a resident, are present at the housing complex. Further, a charging system which is installed in the parking area to charge these electric vehicles should have a function capable of efficiently charging many electric vehicles. [0007] As a charging system in the related art having such a function, a charging control device as disclosed in Patent Document 1 is known, for example. This charging control device individually controls charging, from an external power source of electric storage devices which are respectively mounted on a plurality of vehicles, and includes: an electric storage state detecting unit which detects the electric storage state of the electric storage device when each vehicle and the external power source are connected; an estimated consumed power calculating unit which detects the estimated consumed power amount for each of the plurality of vehicles; a necessary charging power amount calculating unit which calculates the necessary charging power amount on the basis of the detected electric storage state and the estimated consumed power amount for each vehicle; a usage time detecting unit which detects the usage start time of each vehicle; a charging schedule creating unit which determines a charging schedule relating to the charging time and the charging power amount of each vehicle from the necessary charging amount and the usage start time; and a control unit which charges the electric storage device mounted on the vehicle on the basis of the charging schedule. [0008] That is, a preferential electric vehicle can be determined from the necessary charging power and the charging time for the plurality of electric vehicles to create the charging schedule. Related Art Documents Patent Documents [0009] Patent Document': JP-A-2XS-'3c: D9 Summary of the Invention Problem to be Solved by the Invention [0010] However, since electric power supplied to a housing complex from an electric power company is limited to an allowable amount of electric power determined by a contract, if the electric power usage of the entire housing complex exceeds the allowable electric power In a case where many parked electric vehicles are simultaneously charged, the electric power supplied from the electric power company may be cut off, or electricity prices for the entire housing complex may become high depending on the contract with the electric power company. [0011] Further, in a case where the electric power supplied from the electric power company to charging devices used for charging the electric vehicles is limited to the allowable electric power determined by the contract, if the electric power used for charging exceeds the allowable electric power in a case where plural parked electric vehicles are simultaneously charged, the electric power supplied from the electric power company may be cut off, or electricity prices for charging may become high depending on the contract with the electric power company. [0012] In order to prevent these problems, it is necessary to perform adjustments to reduce the electric power used for charging the electric vehicles so as not to exceed the allowable electric power. [0013] Further, for the housing complex where car sharing is performed, it is necessary to efficiently charge electric vehicles individually owned by residents and electric vehicles shared by the'plurality of residents so that the residents are not inconveniently affected in their daily lives. For example, the priority of charging is determined according to living patterns of residents, usage of shared vehicles, environments of electric power available for charging, charging states of respective electric vehicles, and the like. Such a disclosure is not shown in Patent Document 1. [0014] Accordingly, an object of the present invention is to provide a charging system and a charging system control method which are capable of performing efficient charging with limited supplied electric power when a plurality of parked electric vehicles are simultaneously charged. Means for Solving the Problem [0015] A charging system of the present invention includes: parked car number detecting means for, in a case where vehicles parked in a parking area are divided into a first vehicle group owned by an individual and a second vehicle group shared by a plurality of persons, detecting the number of parked cars or a parking rate of the first vehicle group; and a charging control device configured to charge electric vehicles in the first vehicle group and the second vehicle group. The charging control device includes priority determining means for determining the priority of which one of the first vehicle group and the second vehicle group is preferentially charged according to the number of parked cars or the parking rate detected by the parked car number detecting means, and charging electric power supplying means for charging the electric vehicles according to the priority determined by the priority determining means. [0016] According to this configuration, since the priority of which one of the first vehicle group and the second vehicle group is preferentially charged is determined according to the number of the parked cars or the parking rate of the first vehicle group and the electric vehicles are charged according to the determined priority, when the first vehicle group and the second vehicle group are connected to the respective charging devices, it is possible to perform efficient charging with limited supplied electric power. [0017] In the above configuration, the priority determining means preferentially charges the electric vehicles of the first vehicle group when the number of parked cars or the parking rate of the first vehicle group detected by the parked car number detecting means is a predetermined value or more, and preferentially charges the electric vehicles of the second vehicle group when the number of parked cars or the parking rate of the first vehicle group is smaller than the predetermined value. [0018] According to this configuration, since the electric vehicles of the first vehicle group are preferentially charged when the number of parked cars or the parking rate of the first vehicle group is the predetermined value or more and the electric vehicles of the second vehicle group are preferentially charged when the number of parked cars or the parking rate of the first vehicle group is smaller than the predetermined value, the charging priority in the first vehicle group and the second vehicle group is switched according to the number of parked cars or the parking rate of the first vehicle group, and thus, it is possible to perform efficient charging with limited supplied electric power. [0019] A charging system of the present invention includes: a charging control device configured, in a 'case where vehicles parked in a parking area is divided into a first vehicle group owned by an individual and a second vehicle group shared by a plurality of persons, to charge electric vehicles in the first vehicle group and the second vehicle group. The charging control device includes: available electric power measuring means for measuring electric power available for charging the electric vehicles; priority determining means for determining the priority of which one of the first vehicle group and the second vehicle group is preferentially charged according to the available electric power measured by the available electric power measuring means; and charging electric power supplying means for charging the electric vehicles according to the priority determined by the priority determining means. [0020] According to this configuration, since the electric power available for charging the electric vehicles is measured, the priority of which one of the first vehicle group and the second vehicle group is preferentially charged is determined according to the measured available electric power, and the electric vehicles are charged according to the determined priority, when the first vehicle group and the second vehicle group are connected to the respective charging devices, it is possible to perform efficient charging with limited supplied electric power. [0021] In the above configuration, the priority determining means preferentially charges the electric vehicles of the first vehicle group when the available electric power measured by the available electric power measuring means is a predetermined value or more, and preferentially charges the electric vehicles of the second vehicle group when the available electric power measured by the available electric power measuring means is smaller than the predetermined value. [0022] According to this configuration, since the electric vehicles of the first vehicle group are preferentially charged when the available electric power is the predetermined value or more and the electric vehicles of the second vehicle group are preferentially charged when the available electric power is smaller than the predetermined value, the charging priority in the first vehicle group and the second vehicle group is switched according to the available electric power, and thus, it is possible to perform efficient charging with limited supplied electric power [0023] A charging system of the present invention includes: a charging control device configured, in a case where vehicles parked in a parking area are divided into a first vehicle group owned by an individual and a second vehicle group shared by a plurality of persons, to charge electric vehicles in the first vehicle group and the second vehicle group; and timer means for counting the time or day. The charging control device includes: priority determining means for determining the priority of which one of the first vehicle group and the second vehicle group is preferentially charged according to the time or day counted by the timer means; and charging electric power supplying means for charging the electric vehicles according to the priority determined by the priority determining means. [0024] According to this configuration, since the priority of which one of the first vehicle group and the second vehicle group is preferentially charged is determined according to the counted time or day and the electric vehicles are charged according to the priority, when the first vehicle group and the second vehicle group are connected to the respective charging devices, it is possible to perform efficient charging with limited supplied electric power [0025] In the above configuration, the charging system further includes at-home detecting means for detecting an at-home state of an owner for each electric vehicle of the first vehicle group, and the priority determining means charges the electric vehicle of which the owner is at home more preferentially than the electric vehicle of which the owner is not at home. [0026] According to this configuration, since the at-home state of the owner for each electric vehicle of the first vehicle group is detected and the electric vehicle of which the owner is at home is charged more preferentially than the electric vehicle of which the owner is not at home, when the first vehicle group and the second vehicle group are connected to the respective charging devices, it is possible to perform efficient charging with limited supplied electric power [0027] In the above configuration, the charging system further includes storage means for storing a detection result of the at-home detecting means in association with vehicle information on the electric vehicle of the owner. When the first vehicle group is being charged, the priority determining means checks vehicle information obtained from the electric vehicle to be charged with the vehicle information stored in the storage means, obtains the detection result of the at-home detecting means corresponding to the vehicle information in a case where the corresponding vehicle information is present, and charges the electric vehicle of which the owner is at home more preferentially than the electric vehicle of which the owner is not at home. [0028] According to this configuration, since the at-home state of the owner for each electric vehicle of the first vehicle group is detected; the detection result and the vehicle information on the electric vehicle of the owner are correspondingly stored; and when the first vehicle group is being charged, the vehicle information obtained from the electric vehicle to be charged is checked with the stored vehicle information, the detection result of the at-home detection corresponding to the vehicle information is obtained in a case where the corresponding vehicle information is present, and the electric vehicle of which the owner is at home is charged more preferentially than the electric vehicle of which the owner is not at home, when the first vehicle group and the second vehicle group are connected to the respective charging devices, it is possible to perform efficient charging with limited supplied electric power [0029] In the above configuration, the charging control device includes notifying means for notifying the owner of the electric vehicle of the first electric vehicle group, and the priority determining means performs notification of availability of the second vehicle group using the notifying means when the electric vehicle of the second vehicle group is charged more preferentially than the first vehicle group. [0030] According to this configuration, since the availability of the second vehicle group is notified when the electric vehicle of the second vehicle group is charged more preferentially than the first vehicle group, the owner of the electric vehicle in the first vehicle group can be informed that the second vehicle group (shared vehicle) is completely charged and available even though the electric vehicle of the owner is not charged, for example. [0031] A method for controlling a charging system of the invention, the charging system including parked car number detecting means for, in a case where vehicles parked in a parking area are divided into a first vehicle group owned by an individual and a second vehicle group shared by a plurality of persons, detecting the number of parked cars or a parking rate of the first vehicle group, and a charging control device configured to charge electric vehicles in the first vehicle group and the second vehicle group, includes: a priority determining step of determining the priority of which one of the first vehicle group and the second vehicle group is preferentially charged according to the number of the parked cars or the parking rate detected by the parked car number detecting means; and a charging electric power supplying step of charging the electric vehicles according to the priority determined in the priority determining step. [0032] According to this method, since the priority of which one of the first vehicle group and the second vehicle group is preferentially charged is determined according to the number of the parked cars or the parking rate of the first vehicle group and the electric vehicles are charged according to the priority, when the first vehicle group and the second vehicle group are connected to the respective charging devices, it is possible to perform efficient charging with limited supplied electric power. [0033] According to a fifth aspect of the invention, there is provided a program for causing a computer to execute the method for controlling the charging system. [0034] According to this program, when the first vehicle group and the second vehicle group are connected to the respective charging devices, it is possible to perform efficient charging with limited supplied electric power Advantages of the Invention [0035] According to the present invention, when a plurality of parked electric vehicles are simultaneously charged, it is possible to perform efficient charging with limited supplied electric power. Brief Description of the Drawings [0036] Fig. 1 is a diagram schematically illustrating a configuration of a charging system and its periphery according to a first embodiment of the present invention. Fig. 2 is a block diagram schematically illustrating a configuration of the charging system according to the first embodiment of the present invention. Fig. 3 is a diagram illustrating characteristics of the charging system in Fig. 2. Fig. 4 is a flowchart illustrating an operation of the charging system in Fig. 2. Fig. 5 is a flowchart illustrating an operation of the charging system in Fig. 2. Fig. 6 is a block diagram schematically illustrating a charging system according to a second embodiment of the present invention. Fig. 7 is a diagram illustrating characteristics of the charging system in Fig. 6. Fig. 8 is a flowchart illustrating an operation of the charging system in Fig. 6. Fig. 9 is a flowchart illustrating an operation of the charging system in Fig. 6. Fig. 10 is a block diagram schematically illustrating a configuration of a charging system according to a third embodiment of the present invention. Fig. 11 is a diagram illustrating characteristics of the charging system in Fig. 10. Fig. 12 is a flowchart illustrating an operation of the charging system in Fig. 10. Fig. 13 is a flowchart illustrating an operation of the charging system in Fig. 10. Fig. 14 is a block diagram schematically illustrating a configuration of a charging system according to a fourth embodiment of the present invention. Fig. 15 is a diagram illustrating characteristics of the charging system in Fig. 14. Fig. 16 is a flowchart illustrating an operation of the charging system in Fig. 14. Fig. 17 is a flowchart illustrating an operation of the charging system in Fig. 14. Fig. 18 is a block diagram schematically illustrating a configuration of a charging system according to a fifth embodiment of the present invention. Fig. 19 is a diagram illustrating characteristics of the charging system in Fig. 18. Fig. 20 is a diagram illustrating characteristics of an application example of the charging system in Fig. 18. Fig. 21 is a diagram illustrating characteristics of an application example of the charging system in Fig. 18. Fig. 22 is a diagram illustrating characteristics of an application example of the charging system in Fig. 18. Fig. 23 is a flowchart illustrating an operation of the charging system in Fig. 18. Mode for Carrying Out the invention [0037] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. [0038] (First embodiment) Fig. 1 is a diagram schematically illustrating a configuration of a charging system and its periphery according to a first embodiment of the present invention. In the figure, a charging system 401 is installed at a housing complex and charges electric vehicles using electric power supplied from an electric power company 402 and electric power generated by a solar power generation device 403 which is installed on a roof or the like of the housing complex [0039] In order to charge the electric vehicles, the charging system 401 includes a charging device group 404 which is provided to charge shared cars (M) used by residents for car sharing at the housing complex and charges electric vehicles which are parked in a car sharing parking area, and a charging device group 405 which charges electric vehicles (N) owned by individual residents parked in an individual parking area at the housing complex (generally, M