BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a yarn winding device that winds a yarn unwound from a yarn feeding bobbin, and to a yarn unwinding method. 2. Description of the Related Art Automatic winders that form a package by unwinding a yarn produced by a spinning machine or the like from a yarn feeding bobbin are known in the art. Such automatic winders join yarn ends of yarns coming from a plurality of yarn feeding bobbins while removing a yarn defect, such as a slub, and wind the yarn to form the package. Japanese Patent Application Laid-open No. H10-29765 discloses an automatic winder that includes a yarn unwinding assisting device that regulates a bulge of a curved path (also referred to as "balloon") of an unwound yarn to stabilize a yarn tension during unwinding to prevent yarn breakage. The yarn unwinding assisting device disclosed in Japanese Patent Application Laid-open No. H10-29765 includes a movable cylinder put on to cap a top end portion (end portion on a side from which the yarn is unwound) of a yarn feeding bobbin supported upright. The movable cylinder is moved down to keep up with advancement of yarn unwinding from the yarn feeding bobbin. This movable cylinder regulates the bulge of the curved path of the yarn unwound from the yarn feeding bobbin so as not to exceed a predetermined extent. Accordingly, the shape of the balloon is stabilized, so that the yarn unwinding tension is stabilized, and yarn breakage is prevented. Meanwhile, with an aim to increase package production efficiency, unwinding a yarn from a yarn feeding bobbin at a higher winding speed has been desired in recent years. However, if the winding speed (yarn unwinding speed) is increased, the balloon does not bulge out sufficiently during yarn unwinding. Particularly, the balloon is undesirably small when the amount of the yarn on the yarn feeding bobbin is equal to or smaller than one-third a fully wound amount. When the balloon is small, a phenomenon known as sloughing is likely to occur. Sloughing refers to multiple coils of yarn being pulled off at once from the yarn feeding bobbin due to fuzz entanglement. Sloughing results in frequent yarn breakage. As described above, simply increasing the winding speed can result in frequent yarn breakage due to sloughing, thereby undesirably decreasing the package production efficiently. SUMMARY OF THE INVENTION It is an object of the present invention to provide a yarn winding device capable of minimizing occurrences of sloughing during unwinding from a yarn feeding bobbin even when yarn winding is performed at a high speed, thereby increasing package production efficiency. According to an aspect of the present invention, a yarn winding device includes a bobbin support section that supports a yarn feeding bobbin having a core tube onto which a yarn is wound; a yarn winding section that unwinds the yarn from the yarn feeding bobbin supported by the bobbin support section to form a package; and a yarn unwinding assisting device that assists unwinding of the yarn from the yarn feeding bobbin, the yarn unwinding assisting device includes a cylinder put on to cap an end portion of the yarn feeding bobbin, the end portion being on a side from which the yarn is unwound, to regulate a bulge of a curved path of the yarn being unwound from the yarn feeding bobbin. An inner diameter of the cylinder is 8 mm to 11 mm greater than an outer diameter of the core tube of the yarn feeding bobbins to be supported by the bobbin support section, or is equal to 28 mm and less than or equal to 30 mm. According to another aspect of the present invention, a yarn unwinding method for unwinding a yarn from a yarn feeding bobbin that includes a core tube, onto which the yarn is wound, includes preparing a cylinder having an inner diameter greater than or equal to 28 mm and less than or equal to 30, and a core tube having an outer diameter greater than or equal to 17 mm and less than or equal to 22 mm; and arranging the cylinder to cap an end portion of the core tube, the end portion being on a side from which the yarn is unwound, with the cylinder and the yarn feeding bobbin coaxially situated to regulate a bulge of a curved path of the yarn unwound from the yarn feeding bobbin using the cylinder. The above and other objects, features, advantages and the technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of one winding unit of an automatic winder according to an embodiment of the present invention; FIG. 2 is a side view of a yarn unwinding assisting device; FIG. 3 is a front view of the yarn unwinding assisting device; FIG. 4 is a perspective view of a fixed restricting member and a movable cylinder; FIGS. 5A to 5C are diagrams for explaining posture adjustment of a yarn feeding bobbin; FIG. 6 is an external view of a yarn feeding bobbin used in an example; FIG. 7 is a graph showing the number of occurrences of sloughing with a bobbin A of the example; FIG. 8 is a graph showing the number of occurrences of sloughing with a bobbin B of the example; FIG. 9 is a front view of a yarn unwinding assisting device according to a modification; and FIGS. 10A and 10B are enlarged views of a bobbin support section according to another modification. DETAILED DESCRIPTION OF EMBODIMENTS Exemplary embodiments of the present invention are explained below. An exemplary embodiment applied to an automatic winder that includes a number of winding units (yarn winding devices) is described below. Each of the winding units winds a yarn unwound from a yarn feeding bobbin onto a winding tube to form a package. The automatic winder includes a number of winding units, each of which forms a package, arranged in a row in one direction. FIG. 1 is a side view of one of the winding units 1 of the automatic winder. Meanwhile, the right side in FIG. 1, from where an operator operates the winding unit 1, is defined as a front (forward) side, while the left side, which is opposite to the front side, is defined as a back (rear) side. As shown in FIG. 1, each of the winding units 1 includes a bobbin supplying device 2, a bobbin support section 3, a yarn winding section 4, and a unit controller 5 that controls these sections of the winding unit 1. The winding unit 1 winds a spun yarn Y unwound from a yarn feeding bobbin 8 onto a winding tube 6 while causing the spun yarn Y to traverse to thereby form a package P of a predetermined shape. The yarn feeding bobbin 8 is supported by the bobbin support section 3 and the yarn feeding bobbin 8 is supplied to the bobbin support section 3 by the bobbin supplying device 2. The bobbin supplying device 2 houses a plurality of the cylindrical yarn feeding bobbins 8. Each of the yarn feeding bobbins 8 is formed by winding a yarn onto a core tube 9. The bobbin supplying device 2 supplies the yarn feeding bobbins 8 to the bobbin support section 3 one by one. More specifically, the bobbin supplying device 2 includes a columnar magazine 10 that houses a plurality of the cylindrical yarn feeding bobbins 8, and a guide chute 11 arranged below the magazine 10. The guide chute 11 causes one yarn feeding bobbin 8 housed in the magazine 10 to fall onto the bobbin support section 3 while guiding the yarn feeding bobbin 8. A support frame 12 is detachably attached to a machine base 7 of the winding unit 1 with, for example, screws. Each of the magazine 10 and the guide chute 11 is attached to the support frame 12 in a posture slightly inclined toward the front side with respect to the vertical direction so as to face a peg 16 of the bobbin support section 3. The magazine 10 is rotatable about a rotation shaft 13 attached to the support frame 12. A plurality of circumferentially arranged not shown bobbin slots, each capable of housing one yarn feeding bobbin 8, is defined in the magazine 10. A bobbin receiving plate 14 that receives the yarn feeding bobbins 8 housed in the bobbin slots is arranged below the magazine 10. A not shown through-hole is defined in the bobbin receiving plate 14. The bobbin receiving plate 14 is configured such that when one yarn feeding bobbin 8 among the yarn feeding bobbins 8 in the bobbin slots is positioned above the through-hole by rotation of the magazine 10, that yarn feeding bobbin 8 falls in the guide chute 11 via the through-hole. The guide chute 11 includes a pair of opening/closing members 15 that are laterally paired. When the opening/closing members 15 are in an open state, the guide chute 11 guides the yarn feeding bobbin 8 falling from the magazine 10 to the bobbin support section 3. In contrast, when the opening/closing members 15 are in a closed state, the guide chute 11 prevents the yarn feeding bobbin 8 from falling from the magazine 10 to the bobbin support section 3. The bobbin support section 3 includes the peg 16 and a flap plate 17. The peg 16 supports the yarn feeding bobbin 8 by being inserted into a bottom end portion of the core tube 9 of the bobbin 8. The flap plate 17 discharges the yarn feeding bobbin 8 held by the peg 16 out of the bobbin support section 3. The peg 16 includes a first holding piece 16a, which is a projecting piece, and a second holding piece 16b, which is a projecting piece shorter than the first holding piece 16a. The first holding piece 16a and the second holding piece 16b are swingable forward and backward. The second holding piece 16b is also pivotable relative to the first holding piece 16a. Thus, the first holding piece 16a and the second holding piece 16b have an openable/closable structure. When the yarn feeding bobbin 8 is supplied from the bobbin supplying device 2, the peg 16 is inclined toward the front with the first holding piece 16a and the second holding piece 16b in the closed state so that these two holding pieces 16a and 16b are inserted into the bottom end. portion of the core tube 9 of the yarn feeding bobbin 8 that has obliquely fallen from the magazine 10 by being guided by the guide chute 11. Subsequently, the second holding piece 16b is pivoted relative to the first holding piece 16a to bring the holding pieces 16a and 16b into an open state, and the holding pieces 16a and 16b are further pivoted toward the back side. As a result, the yarn feeding bobbin 8 is held in an upright posture as shown in FIG. 1. This upright posture of the yarn feeding bobbin 8 is, as mentioned later, a reference posture during yarn unwinding. The flap plate 17 is on standby in a horizontal state shown in FIG. 1 when the yarn feeding bobbin 8 is held in the upright state by the peg 16. In this state, the flap plate 17 is in contact with the bottom end of the yarn feeding bobbin 8. The flap plate 17 is pivoted toward the front from this state to toss the yarn feeding bobbin 8 mounted on the flap plate 17 forward, thereby discharging the yarn feeding bobbin 8. Meanwhile, pivoting motion of the peg 16 and pivoting motion of the flap plate 17 described above are realized by a support-section motor 18. The driving of the support-section motor 18 is controlled by the unit controller 5. A stepping motor can be used as the support-section motor 18. A yarn unwinding assisting device 20, a tension applying device 21, a yarn joining device 22, a clearer 23, and a waxing device 24 are arranged in this order from the side of the bobbin support section 3 on a yarn path between the bobbin support section 3 and the yarn winding section 4. The yarn unwinding assisting device 20 moves down a movable cylinder (cylinder) 31, which is put on to cap a top end portion of the yarn feeding bobbin 8, to keep up with advancement of unwinding of the yarn Y to thereby regulate a bulge of a curved path (balloon) of the yarn Y being unwound. This results in stabilizing an unwinding tension. The configuration of the yarn unwinding assisting device 20 is explained in detail later. The tension applying device 21 applies a predetermined tension to the running yarn Y. As the tension applying device 21, for example, a gate type tension applying device can be used. The gate type tension applying device has fixed comb teeth and movable comb teeth arranged to be movable relative to the fixed comb teeth. The yarn joining device 22 joins a yarn (lower yarn) coming from the side where the yarn is fed and a yarn (upper yarn) coming from the side where the yarn is wound when the yarn is cut upon detection of a yarn defect by the clearer 23, which will be described next, or when a yarn breakage occurs during yarn unwinding from the yarn feeding bobbin 8. As the yarn joining device 22, for example, an air-type yarn joining device (air splicer) can be used. The air splicer includes untwisting nozzles that untwist an end of the upper yarn and an end of the lower yarn, respectively, and a twisting nozzle that causes a swirling airflow to act on the two untwisted yarn ends to thereby twist the yarn ends together. The clearer 23 detects a yarn defect, such as a slub. A cutter that cuts a yarn when a yarn defect is detected is attached to the clearer 23. The waxing device 24 applies wax to the yarn Y. A lower-yarn catching-and-guiding member 25 that sucks and catches the lower yarn from the bobbin 8 and guides the lower yarn to the yarn joining device 22 is arranged below the yarn joining device 22. An upper-yarn catching-and-guiding member 26 that sucks and catches the upper yarn from the package P and guides the upper yarn to the yarn joining device 22 is arranged above the yarn joining device 22. The upper-yarn catching-and-guiding member 26 is configured like a pipe and supported so as to be pivotable up and down about a shaft 26a. The upper-yarn catching-and-guiding member 26 has, at its leading end, a mouth 26b. Similarly, the lower-yarn catching-and-guiding member 25 is also configured like a pipe and supported so as to be pivotable up and down about a shaft 25a. The lower-yarn catching-and-guiding member 25 has, at its leading end, a suction port 25b. A not shown negative pressure source is connected to each of the lower-yarn catching-and-guiding pipe 25 and the upper-yarn catching-and-guiding member 26 so that air is sucked through the mouth 26b and the suction port 25b to catch the yarn ends of the upper yarn and the lower yarn. The yarn winding section 4 includes a cradle 27 and a traversing drum 28. The cradle 27 includes a pair of cradle arms that rotatably and detachably supports the winding tube 6. The traversing drum 28 can come into contact with a surface of the winding tube 6 supported by the cradle 27 or a surface of the package P formed on the winding tube 6. The yarn winding section 4 rotates the traversing drum 28, which is in contact with the winding tube 6 (or the surface of the package P), using a not shown drum driving motor, thereby causing the winding tube 6 to be rotated (rotated in a linked manner) by rotation of the traversing drum 28 while causing the yarn Y to traverse so that the package P is formed on the outer peripheral surface of the winding tube 6. The configuration of the yarn unwinding assisting device 20 is explained in detail below. FIG. 2 is a side view of the yarn unwinding assisting device 20. FIG. 3 is a front view of the yarn unwinding assisting device 20. Meanwhile, FIG. 3 includes a partially cut¬away view to make the diagram easier to understand. As shown in FIGS. 1 to 3, the yarn unwinding assisting device 20 includes a fixed restricting member 30 fixed to the machine base 7, the movable cylinder 31 that is movable up and down relative to the fixed restricting member 30, a chase-portion detecting sensor 32 (detecting section) that detects a chase portion 8a (a tapered yarn layer portion on the top end portion from which the yarn is unwound) of the yarn feeding bobbin 8, and an ascending/descending mechanism (moving mechanism) 33 that moves the movable cylinder 31 up and down. As shown in FIGS. 1 to 3, the fixed restricting member 30 and the movable cylinder 31 are positioned above the yarn feeding bobbin 8 held in the upright posture by the bobbin support section 3. FIG. 4 is a perspective view of the fixed restricting member 30 and the movable cylinder 31. The fixed restricting member 30 is fixed to the machine base 7 of the winding unit. A restricting portion 30a that regulates a yarn path of the yarn unwound from the yarn feeding bobbin 8 is arranged on a bottom end of the fixed restricting member 30. The movable cylinder 31 includes a straight cylinder portion 31a having a uniform internal diameter in a cylinder-axis direction and a tapered cylinder portion 31b that extends from a bottom end of the straight cylinder portion 31a and has a diameter that gradually increases toward its bottom. The straight cylinder portion 31a of the movable cylinder 31 surrounds the fixed restricting member 30. The restricting portion 30a of the fixed restricting member 30 is housed inside the straight cylinder portion 31a of the movable cylinder 31. The movable cylinder 31 is put on to cap the top end portion of the yarn feeding bobbin 8 such that the movable cylinder 31 and the core tube 9 of the yarn feeding bobbin 8 that is in the upright posture (reference posture during unwinding) are coaxially situated. As described above, in the present embodiment, the configuration that causes the yarn feeding bobbin 8 to obliquely fall from the magazine 10 to the bobbin support section 3 via the guide chute 11 is employed as the bobbin supplying device 2. Therefore, the fixed restricting member 30 and the movable cylinder 31 are configured to guide a yarn end of a yarn coming from the yarn feeding bobbin 8 to insides of the fixed restricting member 30 and the movable cylinder 31 when the yarn feeding bobbin 8 falls on the peg 16 of the bobbin support section 3. More specifically, as shown in FIG. 4, the fixed restricting member 30 includes a guide portion 30b that guides the yarn end to the restricting portion 30a. The movable cylinder 31 has a slit 31c that extends along the entire length from the straight cylinder portion 31a to the tapered cylinder portion 31b. The movable cylinder 31 also includes a guide portion 31d on the straight cylinder portion 31a. The guide portion 31d protrudes outward from an edge of the slit 31c to guide the yarn end to the slit 31c. This configuration causes the yarn end of the yarn from the falling yarn feeding bobbin 8 to be guided through the slit 31c of the movable cylinder 31 to inside of the movable cylinder 31 by a not shown yarn-bringing lever and the guide portion 31d and, furthermore, to the restricting portion 30a of the fixed restricting member 30 housed in the straight cylinder portion 31a. As shown in FIG. 3, a pair of cover members 34, which are laterally paired, is mounted on a side surface of the movable cylinder 31 such that the top end portion of the yarn feeding bobbin 8 is positioned therebetween. The chase-portion detecting sensor 32 is a transmission-type photosensor that includes a light-emitting element 32a and a light-receiving element 32b. The light-emitting element 32a is arranged on one of the cover members 34, and the light-receiving element 32b is arranged on other of the cover members 34 facing each other. As can be seen in FIG. 2, an optical path extending from the light-emitting element 32a to the light-receiving element 32b passes through a position slightly outside the core tube 9 of the uprightly held yarn feeding bobbin 8. Only when a yarn is wound on the core tube 9, light is shielded by a yarn layer on the core tube 9, causing the chase-portion detecting sensor 32 to detect the yarn layer (the chase portion 8a). Meanwhile, as shown in FIG. 1, a result of detection (detection signal relating to the chase portion 8a) output from the chase-portion detecting sensor 32 is transmitted to the unit controller 5. The ascending/descending mechanism 33 that moves the movable cylinder 31 up and down includes an ascending/descending motor 35 (driving section), which is a stepping motor, and a screw mechanism 36 (transmission mechanism) that transmits a driving force of the ascending/descending motor 35 to the cover members 34. The screw mechanism 36 is directly coupled to a drive shaft of the sending/descending motor 35. The screw mechanism 36 includes a vertically extending threaded shaft 37 and a nut member 38 that is screwed onto the threaded shaft 37 and coupled to the cover members 34. A vertically extending guide shaft 39 is inserted through the nut member 38. When the threaded shaft 37 is rotated by the driving force of the ascending/descending motor 35, the nut member 38 moves up and down along the guide shaft 39, causing the cover members 34 and the movable cylinder 31 coupled to the nut member 38, and the chase-portion detecting sensor 32 attached to the cover members 34 to move up and down integrally. As shown in FIG. 1, the unit controller 5 controls the ascending/descending motor 35 based on the result of detection (as to whether the chase portion 8a has been detected) output from the chase-portion detecting sensor 32. More specifically, when the chase-portion detecting sensor 32 cannot detect the yarn layer of the chase portion 8a anymore because yarn unwinding has advanced, the unit controller 5 controls the ascending/descending motor 35 to move down the movable cylinder 31 in the cylinder-axis direction until the chase-portion detecting sensor 32 again detects the chase portion 8a. In the yarn unwinding assisting device 20 described above, winding (yarn unwinding) is performed in a state where the movable cylinder 31 has been moved down to cap the top end portion of the yarn feeding bobbin 8. Accordingly, as shown in FIGS. 1 to 3, the movable cylinder 31 prevents a balloon of the yarn Y unwound from the chase portion 8a from bulging to an extent exceeding a predetermined extent. Therefore, a yarn tension of the unwound yarn is stabilized. In addition, the unwound yarn is smoothly guided into the straight cylinder portion 31a because the tapered cylinder portion 31b is prepared on the bottom end portion of the straight cylinder portion 31a of the movable cylinder 31. This leads to further stabilization of the yarn unwinding tension and reduction in hairiness. In the present embodiment, the movable cylinder 31 is moved in the cylinder-axis direction by the ascending/descending mechanism 33 to keep up with advancement of yarn unwinding from the yarn feeding bobbin 8. Accordingly, the balloon can be formed in a uniform shape by securing a fixed clearance between the chase portion 8a of the yarn feeding bobbin 8 and the movable cylinder 31, thereby achieving stable yarn unwinding. Furthermore, the restricting portion 30a of the fixed restricting member 30 regulates the yarn path so as to be always at the same position, and the balloon having its base point at the restricting portion 30a is regulated by the movable cylinder 31. Accordingly, the shape of the balloon is further stabilized. In addition, the driving section (the ascending/descending motor 35) is dedicated for moving the movable cylinder 31. Accordingly, movement of the movable cylinder 31 can be controlled independently without bothering about operations of other operating sections of the winding unit 1. Furthermore, using the stepping motor as the driving section of the movable cylinder 31 makes it possible to control the movement of the movable cylinder 31 easily by adjusting the number of drive pulses of the stepping motor. Furthermore, damage is prevented even in case of a trouble where the movable cylinder 31 should become unmovable by being caught by another member because the motor 35 goes out of step in such a case. Furthermore, movement of the movable cylinder 31 can be controlled accurately using the screw mechanism 36 as the transmission mechanism that transmits the driving force of the ascending/descending motor 35. Furthermore, another advantage is that the current state can be maintained even if the driving force of the ascending/descending motor 35 should be lost. Meanwhile, to increase production efficiency of the package P of each of the winding units 1 in the automatic winder, it is desirable to increase a yarn winding speed (e.g., to 1,700 m/min or higher) of the yarn winding section 4. However, when a yarn unwinding speed from the yarn feeding bobbin 8 is increased, the balloon may not bulge out sufficiently because of a small yarn layer, and the yarn may be unwound without forming a balloon. In this case, the yarn that has been just unwound is likely to be rubbed against the surface of the yarn layer, and sloughing is likely to occur, resulting in frequent yarn breakages. This situation is likely to occur when, particularly, an amount of the yarn on the yarn feeding bobbin 8 is equal to or smaller than one-third a fully wound amount. In view of this fact, an inner diameter d2 of the movable cylinder 31 shown in FIG. 3 is set to a value slightly greater than an outer diameter Dl of the top end portion of the core tube 9 of the yarn feeding bobbin 8 so that the balloon bulges out sufficiently even when yarn winding is performed at a high winding speed. More specifically, for the outer diameter Dl of the top end portion of the yarn feeding bobbin 8 (that is, greater than or equal to 17 mm and less than or equal to 22 mm), the inner diameter d2 of the straight cylinder portion 31a of the movable cylinder 31 is set to greater than or equal to 28 mm and less than or equal to 30 mm. When the inner diameter of the movable cylinder 31 is relatively small, an upper portion of the balloon is narrowed, while the balloon is continuously formed at its lower portion that is closer to the yarn layer. Accordingly, the yarn that has been just unwound is less likely to come into contact with the surface of the yarn layer, and sloughing is prevented. Meanwhile, a configuration of the winding unit 1 is available in which yarn feeding bobbins with the core tubes 9 having different diameters are prepared, and yarn feeding bobbins having a certain diameter are selected for use among those. When this configuration is employed, the movable cylinders 31 having different inner diameters are prepared, and the movable cylinders 31 having an inner diameter appropriate for the selected yarn feeding bobbins 8 is used. An example where two types of the yarn feeding bobbins 8 that differ from each other in outer diameter of the core tube 9 are used is described below. When the outer diameter Dl of the top end portion of the yarn feeding bobbin 8 to be used is greater than or equal to 17 mm and less than or equal to 20 mm, the movable cylinder 31 with an inner diameter 28 mm is used. In contrast, when the outer diameter Dl of the top end portion of the yarn feeding bobbin 8 to be used is greater than 20 mm and equal to or smaller than 22 mm, the movable cylinder 31 with an inner diameter 30 mm is used. By selectively using the movable cylinder 31 based on the diameter of the core tube 9 of the yarn feeding bobbin 8 as described above, occurrences of sloughing during high-speed yarn winding can be reduced irrespective of the diameter of the used yarn feeding bobbins 8. Meanwhile, when the inner diameter of the movable cylinder 31 is small, the clearance between the yarn feeding bobbin 8 and the movable cylinder 31 put on to cap the yarn feeding bobbin 8 is small. When the clearance is small, the movable cylinder 31 is comes into contact with the yarn feeding bobbin 8 even by a slight tilt of the yarn feeding bobbin 8 from the upright posture (the posture where the yarn feeding bobbin 8 and the movable cylinder 31 are coaxially situated) which is the reference posture during yarn unwinding, resulting in defective unwinding. To this end, the winding unit 1 according to the present embodiment further includes a posture adjuster for maintaining the upright posture of the yarn feeding bobbin 8 and causing an axis of the core tube 9 of the yarn feeding bobbin 8 to coincide with an axis L of the movable cylinder 31. In the present embodiment, posture adjustment of the yarn feeding bobbin 8 is performed using the result of detection output from the chase-portion detecting sensor 32. FIGS. 5A to 5C are diagrams for explaining the posture adjustment of the yarn feeding bobbin 8. When the yarn feeding bobbin 8 is supplied by the bobbin supplying device 2 to the bobbin support section 3, the yarn feeding bobbin 8 is held by the peg 16 (see FIG. 1) of the bobbin support section 3. Thereafter, the peg 16 pivots rearward, bringing the yarn feeding bobbin 8 to the upright posture as shown in FIG. 5A. However, in this state, it is possible that the axis of the yarn feeding bobbin 8 is slightly tilted relative to the axis L of the movable cylinder 31. There are several possible reasons for occurrence of such a tilt. One particular reason is low stability of the peg 16 of the present embodiment where the peg 16 holds the yarn feeding bobbin 8 only with the two holding pieces 16a and 16b whereby the posture of the yarn feeding bobbin 8 is likely to fluctuate. To take care of this issue, the axis of the yarn feeding bobbin 8 is caused to coincide with the axis L of the movable cylinder 31 in this state as described below. First, in a state shown in FIG. 5A where posture adjustment of the yarn feeding bobbin 8 is not performed yet, the movable cylinder 31 is on standby above the yarn feeding bobbin 8 (in a state where the movable cylinder 31 is not capping the yarn feeding bobbin 8), where nothing is detected by the chase-portion detecting sensor 32. From this state, the unit controller 5 controls the support-section motor 18 that drives the peg 16 to cause the yarn feeding bobbin 8 to pivot (swing) frontward (to the right side in FIG. 5A). When the chase-portion detecting sensor 32 detects the top end portion of the yarn feeding bobbin 8 as shown in FIG. 5B, the unit controller 5 causes the yarn feeding bobbin 8 to stop pivoting. Meanwhile, the unit controller 5 includes a storing section that stores therein in advance number of pulses required to drive the support-section motor 18 to swing the yarn feeding bobbin 8 rearward (to the left in FIG. 5B) from a position shown in FIG. 5B, at which the chase-portion detecting sensor 32 detects the yarn feeding bobbin 8, to bring the yarn feeding bobbin 8 into the reference posture in which the yarn feeding bobbin 8 and the movable cylinder 31 have a common axis. The unit controller 5 outputs the number of pulses stored in the storing section to the support-section motor 18 and causes the yarn feeding bobbin 8 to swing rearward by an angle corresponding to the number of pulses. Thus, the unit controller 5 can cause the axis of the yarn feeding bobbin 8 to coincide with the axis L of the movable cylinder 31. The axis of the core tube 9 of the yarn feeding bobbin 8 is caused to coincide with the axis L of the movable cylinder 31 and thereafter the movable cylinder 31 is moved down to cap the end portion of the yarn feeding bobbin 8 as described above. Accordingly, even when the clearance between the movable cylinder 31 and the core tube 9 of the yarn feeding bobbin 8 is small, contact between the movable cylinder 31 and the yarn feeding bobbin 8 is less likely to occur. Therefore, the inner diameter of the movable cylinder 31 can be set to greater than or equal to 28 mm and less than or equal to 30 mm, which is only slightly larger than the outer diameter of the core tube 9 of the yarn feeding bobbin 8. Accordingly, even when yarn winding is performed at a high winding speed, the balloon of the unwound yarn bulges out sufficiently, and occurrences of sloughing are reduced. Concrete experiments performed to test effectiveness of the present invention are explained below. Two types of yarn feeding bobbins that differ from each other in diameter of the core tube and the like were prepared. Yarn winding was performed using each of the yarn feeding bobbins and movable cylinders of different inner diameters. Yarn feeding bobbins FIG. 6 is an external view of concrete examples of the yarn feeding bobbins used in this experiment. Yarn types and major dimensions of the two types of yarn feeding bobbins shown in FIG. 6 are given in Table 1. Note that the yarn counts in Table 1 are English yarn counts. Movable cylinders A total of five types of movable cylinders were used. The inner diameters (indicated by d2 in FIG. 3) of the straight cylinder portions of the movable cylinders were 28 mm (hereinafter, this movable cylinder is referred to as "cp28 cylinder "), 30 mm (hereinafter, "cp30 cylinder "), 32 mm (hereinafter, "