BODY COMPOSITION METER BACKGROUND OF THE ART The present invention relates to a body composition measurement device including two current electrodes and two voltage electrodes. A body composition measurement device (body fat measurement device) measures a body composition using differences in the electrical resistance of human body compositions such as fat and muscles. Japanese Laid-Open Patent Publication No. 11-309123 describes an example of a body composition measurement device that incorporates electrodes in a belt. Japanese Laid-Open Patent Publication No. 2001-190513 describes an example of a body composition measurement device including separable housings with electrodes. When measuring the body composition, two voltage electrodes that are used to measure voltage are required to be brought into contact with a predetermined portion of the human body. However, in the body composition measurement device described in Japanese Laid-Open Patent Publication No. 11-309123, depending on the build (abdominal ! circumference) of the user using the body composition measurement device, it may be difficult to accurately measure the body composition. Specifically, as shown in Fig. 11, if a user having a large abdominal circumference uses a body composition measurement device 101, which is described in Japanese Laid-Open Patent Publication No. 11-309123, to measure the body composition at the abdomen 191, two current electrodes 120 and two voltage electrodes 130 are arranged at the front of the human body 109. However, as shown in Fig. 12, if a user having a small abdominal circumference uses the same body-composition measurement device 101 to measure the body composition at the abdomen 191, the current electrodes 120 and the voltage electrodes 130 will be located toward the side of the human body 109. In this manner, the portion of the human body that comes into contact with the voltage electrodes 130 differs between a user having a large abdominal circumference and a user having a small abdominal circumference even when using the same body composition measurement device 101. In other words, the portion of the body on which the electrodes are positioned to measure the body composition differs between users having a large abdominal circumference and a small abdominal circumference. Thus, the voltage electrodes 130 measure the voltage of different portions, and the body composition measurement may not necessarily be accurate. In the body composition measurement device described in Japanese Laid-Open Patent Publication No. 2001-190513, two voltage electrodes may be arranged at any portion of a user. However, the freedom in arrangement may result in the voltage electrodes being arranged at an inappropriate positioned. Accordingly, the body composition measurement device described in Japanese Laid-Open Patent Publication No. 2001-190513 may also be unable to accurately measure the body composition. Accordingly, it is an object of the present invention to provide a body composition measurement device capable of suppressing a region where the electrode is arranged from changing according to the size of the abdominal circumference. SUMMARY OF THE INVENTION One aspect of the present invention is a body composition measurement device including two current electrodes and two voltage electrodes. The body composition measurement device includes a main body, a width measurement unit, and an electrode position adjustment unit. The main body includes a position reference point aligned with a predetermined reference region of a measured portion of a human body. The width measurement unit is arranged on the main body. The width measurement unit is adjustable to a position corresponding to a width of the measured portion based on the alignment of the position reference point and the reference region. The electrode position adjustment unit moves the current electrodes or the voltage electrodes in cooperation with the position adjustment of the width measurement unit. The electrode position adjustment unit moves the voltage electrodes, in cooperation with the position adjustment of the width measurement unit, by a first distance, which is a fixed ratio of a distance in which the width measurement unit is moved, in a direction of a predetermined first angle. Further, the electrode position adjustment unit moves the current electrodes, in cooperation with the position adjustment of the width measurement unit, by a second distance, which is a fixed ratio of the distance in which the width measurement unit is moved, in a direction of a predetermined second angle. Preferably, in the body composition measurement device, the electrode position adjustment unit is configured to link the movement of the voltage electrodes and the movement of the current electrodes in cooperation with the position adjustment of the width measurement unit. Preferably, the body composition measurement device includes a detector that detects contact of the width measurement unit with the measured portion. Preferably, the body composition measurement device includes a meter that measures the distance in which the width measurement unit is moved. Preferably, in the body composition measurement device, the body composition measurement device includes a drive source driven to move the width measurement unit. Preferably, in the body composition measurement device, the reference region is an umbilicus. Preferably, the body composition measurement device includes a sensor that measures subcutaneous fat of the measured portion. Preferably, in the body composition measurement device, the current electrodes are arranged on the width measurement unit, and the voltage electrodes are arranged on the main body. Preferably, in the body composition measurement device, the electrode position adjustment unit is configured to move the current electrodes in a direction opposite to a direction in which the width measurement unit is moved. Preferably, the body composition measurement device includes a biasing member that biases the current electrodes toward the measured portion with a biasing force corresponding to the movement of the current electrodes. The present invention provides a body composition measurement device capable of suppressing a region where the electrode is arranged from changing according to the size of the abdominal circumference. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic showing a body composition measurement device according to a first embodiment when used; Fig. 2 is a schematic block diagram showing the electrical configuration of the body composition measurement device of Fig. 1; Fig. 3 is a schematic perspective view showing the body composition measurement device of Fig. 1; Fig. 4 is a schematic plan view showing the internal structure of the body composition measurement device of Fig. 3; Fig. 5 is a schematic diagram showing movement of a current electrode in the body composition measurement device of Fig. 3; Fig. 6 is a schematic diagram showing the movement of a voltage electrode in the body composition measurement device of Fig. 3; Fig. 7 is a schematic enlarged view showing the internal structure of a right width measurement unit in a body composition measurement device according to a second embodiment; Fig. 8 is a schematic enlarged view showing the internal structure of a main body in a body composition measurement device according to a third embodiment; Fig. 9 is a schematic plan view showing the internal structure of a right portion when dividing a modified body composition measurement device into a left half and a right half; Fig. 10 is a perspective view showing a modified body composition measurement device including a sensor that detects the alignment of an index and the umbilicus; Fig. 11 is a schematic diagram showing a prior art body composition measurement device when used; and Fig. 12 is a schematic view showing the prior art body composition measurement device when used. DETAILED DESCRIPTION OF THE INVENTION [First Embodiment] A body composition measurement device 1 according to a first embodiment will now be described with reference to Figs. 1 to 6. As shown in Fig. 1, a body composition measurement device 1, which is a body fat measurement device, is used in contact with a measured portion of a human body 9. In the present example, the body composition measurement device 1 measures the body fat at an abdomen 91. The body composition measurement device 1 is aligned with, for example, the umbilicus 92 and positioned relative to the abdomen 91. As shown in Fig. 2, the body composition measurement device 1 includes an electrode group 2, a control unit 11, an operation unit 12, and a display unit 13. The electrode group 2 is used to measure the body composition. The control unit 11 controls the measurement of the body composition using the electrode group 2. The operation unit 12 operates the body composition measurement device 1. The display unit 13 shows the measurement results of the body composition. The electrode group 2 includes electrodes 21, 22, 31, and 32. Specifically, the electrode group 2 includes a first current electrode 21 and a second current electrode 22, which serve as current electrodes that generate a flow of current. The first current electrode 21 and the second current electrode 22 form a current electrode pair 20. The current electrodes 21 and 22 are connected to the control unit 11 by electrical wires. The electrode group 2 also includes a first voltage electrode 31 and a second voltage electrode 32, which serve as voltage electrodes for measuring voltage. The first voltage electrode 31 and the second voltage electrode 32 form a voltage electrode pair 30. The voltage electrodes 31 and 32 are connected to the control unit 11 by electrical wires. The control unit 11 is, for example, an integrated circuit formed by a microcomputer. When receiving a signal instructing the measurement of the body fat to be started, the control unit 11 starts the measurement of the body fat. The control unit 11 generates a flow of current to the current electrodes 21 and 22 when starting the measurement of the body fat. The control unit 11 measures the voltage between the voltage electrodes 31 and 32. The control unit 11 measures the body fat at the measured portion based on the measurement result of the voltage. The control unit 11 then sends a signal indicating the measurement result of the body fat to the display unit 13. The operation unit 12 is an input man-machine interface including switches and buttons. The operation unit 12 is connected to the control unit 11 by electrical wires. Operation of the operation unit 12 provides the control unit 11 with a signal for starting the body fat measurement. The display unit 13 is an output man-machine interface formed, for example, by a liquid crystal panel. The display unit 13 is connected to the control unit 11 by electrical wires. The display unit 13 shows the measurement result of the body fat based on the signal indicating the measurement result of the body fat from the control unit 11. The layout of the electrode group 2 in the body composition measurement device 1 will now be described with reference to Fig. 3. As shown in Fig. 3, the body composition measurement device 1 includes a main body 41, which is a generally V-shaped hollow box. The substantially V-shaped main body 41 extends outward at the front and extends inward at the rear as viewed in Fig. 3. The rear side (hereinafter, also referred to as the "inner side") of the main body 41 includes walls defining surfaces 41a, 41b, and 41c. The surface 41c is located toward the front and extends sideward. Surfaces 41a and 41b adjacent to the left and right sides of the surface 41c extend toward the front at angle cp relative to a direction orthogonal to a plane that divides the main body 41 into a left half and a right half. The angle