[0001]The present invention relates to an exhaust gas flow rate measuring unit that is mounted in a vehicle and measures the flow rate of exhaust gas emitted from an exhaust pipe (tail pipe) of the vehicle, and to an exhaust gas analyzing apparatus using the exhaust gas flow rate measuring unit. Background Art [0002] As a conventional vehicle emissions test, a bench test adapted to run a vehicle on a chassis dynamometer in a simulated manner has been performed. In the bench test, the emission mass of each component contained in exhaust gas emitted from an exhaust pipe of the vehicle during the simulation run is calculated by diluting the exhaust gas using a constant volume sampler (CVS), sampling the diluted exhaust gas in a bag, and analyzing the concentration of that component contained in the sampled diluted exhaust gas. [0003] In recent years, in addition to the bench test described above, a real driving emission (RDE) test has also been spreading. As disclosed in Patent Literature 1, in the RDE test, a vehicle is actually run on a road with a vehicle-mounted analyzer mounted therein, and exhaust gas emitted from an exhaust pipe during the on-road run is sampled to analyze the concentration of each component contained in the exhaust gas. Also, the exhaust pipe is installed with, for example, a Pitot tube flowmeter for measuring the flow rate of the exhaust gas, and from the exhaust gas flow rate obtained by the flowmeter and the concentration of each component obtained by the vehicle-mounted analyzer, the emission mass of that component is calculated. In this case, the flowmeter is installed in an attachment pipe that is attached to the exhaust pipe as well as forming the flow path of the exhaust gas emitted from the exhaust pipe. [0004] Also, in order to increase the reliability of the measurement result of the emission mass of each component in the RDE test, a correlation test between emission mass obtained using the vehicle-mounted analyzer and emission mass obtained using a CVS and a bag is performed. In the correlation test, the vehicle mounted with the vehicle-mounted analyzer is run on a chassis dynamometer in a simulated manner, and the emission mass obtained using the vehicle-mounted analyzer during the simulation run and the emission mass obtained using the CVS and the bag are compared. Then, it is detennined whether or not the emission mass obtained using the vehicle-mounted analyzer falls within a predetennined error range (e.g., within ±10 %) centering on the emission mass obtained using the CVS and the bag, and when it is within the predetennined error range, the RDE test is performed on the vehicle mounted with the vehicle-mounted analyzer. [0005] However, even though the vehicle-mounted analyzer and the flowmeter are both normal, when perfonning the correlation test with both mounted in a vehicle, the emission mass obtained using the vehicle-mounted analyzer may fall outside the predetennined enor range centering on the emission mass obtained using the CVS and the bag. This may be mainly because as illustrated in FIG. 5, a disturbance such as the deviation of the flow velocity distribution of the exhaust gas flowing through the flow path of the attachment pipe from the centerline of the flow path causes a measurement enor in the exhaust gas flow rate obtained by the flowmeter. FIG. 5 is a simulation result of the flow velocity distribution of exhaust gas in the flow path of a junction pipe of an attachment pipe having: multiple branch pipes respectively connected to multiple exhaust pipes; and the junction pipe joined by the multiple branch pipes. Note that the disturbance such as the deviation of the flow velocity distribution varies depending on flow velocity as well. The measurement enor also causes a measurement enor in the emission mass of each component. Citation List Patent Literatures [0006] Patent Literature 1: Japanese Unexamined Patent Publication JP-A 2004-144574 Summary of Invention Technical Problem [0007] Therefore, the present invention is made in order to solve the above-described problem, and a main object thereof is to improve the measurement accuracy of the flow rate of exhaust gas flowing through the flow path of an attachment pipe connected to an exhaust pipe of a vehicle by reducing the disturbance of the flow velocity distribution of the exhaust gas. Solution to Problem [0008] That is, an exhaust gas flow rate measuring unit according to the present invention is one that is mounted in a vehicle and measures the flow rate of exhaust gas emitted from an exhaust pipe of the vehicle, and the exhaust gas flow rate measuring unit includes: an attachment pipe that is connected to the exhaust pipe and forms a flow path through which the exhaust gas flows; a flowmeter that is provided in the flow path and measures the flow rate of the exhaust gas flowing through the flow path; and a straightening mechanism that is provided on the upstream side of the flowmeter in the flow path. [0009] hi such an exhaust gas flow rate measuring unit, the straightening mechanism is provided on the upstream side of the flowmeter in the flow path of the attachment pipe, and therefore the disturbance of the flow velocity distribution of the exhaust gas flowing into the flowmeter, such as the deviation of the flow velocity distribution, can be reduced to reduce the measurement error of the flowmeter. Accordingly, the flow rate of the exhaust gas emitted from the exhaust pipe of the vehicle can be accurately measured. [0010] When the flowmeter is a Pitot tube flowmeter, the effect of the present invention can be further distinguished. The Pitot tube flowmeter is one adapted to detect the differential pressure AP between the total pressure and static pressure of the exhaust gas, and includes a Pitot tube having total pressure holes for detecting the total pressure and static pressure holes for detecting the static pressure. The positions of the total and static pressure holes are fixed in the flow path, and therefore when the flow velocity distribution of the exhaust gas is disturbed, the Pitot tube flowmeter is likely to be affected by the disturbance to consequently change the differential pressure AP, thus making it impossible to accurately detect the differential pressure. However, as in the present invention, by providing the straightening mechanism on the upstream side of the Pitot tube flowmeter to eliminate the disturbance of the flow velocity distribution of the exhaust gas, the flow rate of the exhaust gas can be accurately measured even in the case of the Pitot tube flowmeter that is likely to be affected by the flow velocity distribution of the exhaust gas. [0011] It is preferable that the attachment pipe includes a straight pipe part, and the flowmeter and the straightening mechanism are provided in the straight pipe part. This configuration allows the exhaust gas straightened by the straightening mechanism to flow into the flowmeter while keeping the straightened state, and therefore the measurement accuracy of the exhaust gas flow rate can be further improved. Also, when the attachment pipe includes a curved pipe part or a junction part on the upstream side of the straight pipe part, the effect of providing the straightening mechanism can be distinguished. [0012] When the vehicle is one including multiple exhaust pipes, it is preferable that the attachment pipe includes: multiple branch pipes connected to the multiple exhaust pipes; and a junction pipe joined by the multiple branch pipes; and the flowmeter and the straightening mechanism are provided in the junction pipe. This configuration makes it possible to measure the total flow rate of exhaust gases emitted from the multiple exhaust pipes at once. Also, since the exhaust gases from the multiple branch pipes flow into the junction pipe, in the flow path of the junction pipe, the disturbance of the flow velocity distribution of the joined exhaust gases and/or the spiral flow of the joined exhaust gases are likely to occur. However, the straightening mechanism is provided on the upstream side of the flowmeter in the junction pipe, and therefore the measurement accuracy of the exhaust gas flow rate obtained by the flowmeter can be improved. [0013] Further, in order to preferably use the exhaust gas flow rate measuring unit of the present invention for a vehicle-mounted exhaust gas analysis system, it is preferable to further include an exhaust gas sampling part that is provided in the flow path and for sampling the exhaust gas. [0014] Still further, it is preferable that an exhaust gas analyzing apparatus according to the present invention includes: the above-described exhaust gas flow rate measuring unit; and an exhaust gas analyzer that measures the concentration of a predeteirnined component contained in the exhaust gas, and the attachment pipe is provided with an exhaust gas sampling part for sampling the exhaust gas and introducing the sampled exhaust gas to the exhaust gas analyzer. In this exhaust gas analyzing apparatus, the measurement accuracy of the exhaust gas flow rate measured by the exhaust gas flow rate measuring unit can be improved, and therefore the exhaust gas analyzing apparatus can obtain the emission mass of each exhaust gas component with high accuracy to thereby improve the reliability of the RDE test. Advantageous Effects of Invention [0015] According to the present invention configured as described above, since the straightening mechanism is provided on the upstream side of the flowmeter in the flow path of the attachment pipe, the disturbance of the flow velocity distribution of the exhaust gas flowing into the flowmeter can be eliminated to improve the measurement accuracy of the flowmeter. Brief Description of Drawings [0016] FIG. 1 is a schematic view illustrating the configuration of an exhaust gas analyzing apparatus according to the present embodiment; FIG. 2 is a schematic view illustrating the configuration of an exhaust gas flow rate measuring unit in the same embodiment; FIG. 3 is a schematic view illustrating the positional relationship between a flowmeter and a straightening mechanism; FIG. 4 is front and side views illustrating an example of the straightening mechanism; and FIG. 5 is a simulation result of the flow velocity distribution of exhaust gas in an attachment pipe when no straightening mechanism is provided. Description of Embodiments [0017] In the following, one embodiment of an exhaust gas analyzing apparatus using an exhaust gas flow rate measuring unit according to the present invention will be described with reference to drawings. [0018] Since the exhaust gas analyzing apparatus 100 of the present embodiment configured as described above is such that the straightening mechanism 5 is provided on the upstream side of the flowmeter 4 in the flow path R of the attachment pipe 3, the 10 disturbance of the flow velocity distribution of the exhaust gases flowing into the flowmeter 4 such as the deviation of the flow velocity distribution can be reduced to reduce the measurement error of the flowmeter 4. In particular, in the case of the RDE test, it is difficult to sufficiently ensure the length of the straight pipe part of the attachment pipe 3, and therefore providing the straightening mechanism 5 is effective. 15 Accordingly, the flow rate of the exhaust gases emitted from the exhaust pipes EH of the vehicle V can be accurately measured. In doing so, the exhaust gas analyzing apparatus 100 can obtain the emission mass of each exhaust gas component with high accuracy, and the reliability of the RDE test can be improved. 20 [0032] Also, in the correlation test with the bag measurement using a CVS performed before the RDE test, emission mass can be obtained with high accuracy using the vehicle-mounted analyzer (exhaust gas analyzing apparatus 100), and therefore it can be accurately determined whether or not the emission mass is within a predetermined error 25 range (e.g., within ±10 %) centering on emission mass obtained using the CVS and a bag. [0033] Further, since the junction pipe 32 of the attachment pipe 3 is provided with the 13 flowmeter 4, the total flow rate of the exhaust gases emitted from the multiple exhaust pipes EH can be measured by the one flowmeter 4 at once. In addition, since the exhaust gases from the multiple branch pipes 31 flow into the junction pipe 32, the disturbance of the flow velocity distribution of the exhaust gases and/or the spiral flow 5 of the exhaust gases are likely to occur. However, the straightening mechanism 5 is provided on the upstream side of the flowmeter 4 in the junction pipe 32, and therefore the measurement accuracy of the exhaust gas flow rate obtained by the flowmeter 4 can be improved. 10 [0034] Note that the present invention is not limited to the above-described embodiment. 15 [0035] For example, the attachment pipe in the above-described embodiment is one connected to the multiple exhaust pipes, but may be one connected to a single exhaust pipe. 20 [0036] Also, as the flowmeter, in addition to the Pitot tube flowmeter, a flowmeter based on another measurement principle, such as a differential pressure flowmeter such as a flow nozzle, an ultrasonic flowmeter, or a vortex flowmeter may be used. 25 [0037] Further, the attachment pipe may be provided with multiple flowmeters. In this case, by taking the sum or average of exhaust gas flow rates obtained by the multiple flowmeters or by other means, the flow rate of the exhaust gases emitted from the exhaust pipes is calculated. 14 [0038] Still further, the exhaust gas flow rate measuring unit in the above-described embodiment can also be used for, in addition to the RDE test, a development test for vehicles or the like in a laboratory or other setting. [0039] Besides, it goes without saying that the present invention is not limited to any of the above-described embodiments, but can be variously modified without departing the scope thereof Reference Signs List [0040] 100: Exhaust gas analyzing apparatus V: Vehicle EH: Exhaust pipe 2: Exhaust gas flow rate measuring unit 3: Attachment pipe 31: Multiple branch pipes 32: Junction pipe 32x: Straight pipe part 4: Flowmeter 5: Straightening mechanism (upstream side) 6: Straightening mechanism (downstream side) 7: Exhaust gas analyzer 8: Introduction pipe 9: Exhaust gas sampling part WE CLAIM: 1. An exhaust gas flow rate measuring unit that is mounted in a vehicle and measures a flow rate of exhaust gas emitted from an exhaust pipe of the vehicle, the exhaust gas flow rate measuring unit comprising: an attachment pipe that is connected to the exhaust pipe and forms a flow path through which the exhaust gas flows; a flowmeter that is provided in the flow path and measures the flow rate of the exhaust gas flowing through the flow path; and a straightening mechanism that is provided on an upstream side of the flowmeter in the flow path. 2. The exhaust gas flow rate measuring unit as claimed in claim 1, wherein the attachment pipe includes a straight pipe part, and the flowmeter and the straightening mechanism are provided in the straight pipe part. 3. The exhaust gas flow rate measuring unit as claimed in claim 2, wherein the attachment pipe includes a curved pipe part or a junction part on an upstream side of the straight pipe part. 4. The exhaust gas flow rate measuring unit as claimed in claim 1, used for an on-road mnning test of the vehicle. 5. The exhaust gas flow rate measuring unit as claimed in claim 1, wherein the flowmeter is a Pitot tube flowmeter. 6. The exhaust gas flow rate measuring unit as claimed in claim 1, wherein the vehicle is one including multiple exhaust pipes, the attachment pipe includes: multiple branch pipes connected to the multiple exhaust pipes; and a junction pipe joined by the multiple branch pipes; and the flowmeter and the straightening mechanism are provided in the junction pipe. 7. The exhaust gas flow rate measuring unit as claimed in claim 1, further comprising an exhaust gas sampling part that is provided in the flow path and for sampling the exhaust gas. 8. An exhaust gas analyzing apparatus comprising: the exhaust gas flow rate measuring unit as claimed in claim 1; and an exhaust gas analyzer that measures concentration of a predetermined component contained in the exhaust gas, wherein the attachment pipe is provided with an exhaust gas sampling part for sampling the exhaust gas and introducing the sampled exhaust gas to the exhaust gas analyzer. 9. The exhaust gas analyzing apparatus as claimed in claim 8 that is a vehicle- mounted one.