TECHNICAL FIELD The present invention relates to a pulley structure having a pulley member and a hub s t r u c t u r e that can r o t a t e relative t o t h e pulley member. BACKGROUND ART In general, as a transmission mechanism for transmitting the power of an engine of a motor vehicle, there is a belt transmissian mechanism i n which a belt is looped between pulleys. Additionally, as a pullGy structure that is used in I, such a b e l t transmission mechanism, there is known a pu1l.e~ s t r u c t u r e having a configuration for damping a change in rotation when the change in rotation is generated in one of two r o t a t i o n a l members (fox example, Patent Document I ) . As a technique of this kind, Patent Document 1 discloses a pulley including an annular pulley member, a hub s t r u c t u r e (to which a shaft of an alternating c u r r e n t generator is fixed so as not to rotate relative thereto), and a coil spring that is mounted between the annular pulley member and the hub structure. According to t h i s configuration, when a change i n rotation is generated i n the hub structure, the coil spring between the hub structure and the annular p u l l e y member is e l a . s t i c a l l y deformed to thereby damp the change in rotation. However, when the natural frequency of t h e pulley described i n Patent Document 1 above is set to be equal to or smaller than a ftequency that is considered from experience t o be generated by an engine revolution speed when an engine is idling, there may be a situation in which the pulley resonates when the engine s t a r t s to rotate or stops r o t a t i n g . As a r e s u l t , the relative d i s t o r t i o n displacement between the annular pulley member and the hub structure is incxeased abruptly, andan excessive force is a p p l i e d t o t h e c o i l spring, leading to a problem t h a t a f a i l u r e of the coil spring i t s e l f li is called for. To cope with t h i s problem, there has been proposed a pulley structure which adopts a s p r i n g c l u t c h c o n s t r u c t i o n . As raised i n Patent Document 2, for example, i n t h i s pulley structure, anendportionofacoilspringisnotfixeddirectly to a pulley member or a hub s t r u c t u r e . The end portion of the c o i l spring is e l a s t i c a l l y deformed i n a radial direction and is then attached to the pulley member or the hub s t r u c t u r e by virtue of a restoring force of the coil spring. Then, when an i n p u t torque t h a t exceeds a f r i c t i o n a l torque t h a t is generated between the end portion of the coil spring and the pulley member or the hub s t r u c t u r e is applied to the pulley member or the hub. structure, a s l i p is cause to occur between the end portion of the coil spring and the p u l l e y member or the hub structure so as to suppress the application of an excessive force to the coil spring, whereby the breakage of 0 t h e coil spring itself can be prevented. RELATE ART DOCUMENT PATENT DOCUMENT Patent Document 1: Japanese Patent No. 3268007 Patent Document 2 : JP-A-2003-322174 PROBLEMS THAT THE INVENTION IS TO SOLVE When a spring clutch csnstruction like the one described above i s adopted, since t h e coil spring is produced by using a plastic deformation technique, it is d i f f i c u l t to have uniformaccuracyindirnensionandprafile, leadingtoaproblem t h a t a frictional torque generated between the end portion of t h e coil springandthepulleymember or the hub structure tends t o v a r y e a s i l y . Additionally, the frictional torquegenerated between t h e e n d p o r t i o n o f t h e c o i l springandthepulleymember or the hub structure is determined by the material and performance of the coil spring and the pulley member or the hub structure. However, the s e l e c t i o n of a material and a performance fora c o i l s p r i n g t o b e u s e d i s l i m i t e d (the degree I of freedom indesign is lowered) fromtheviewpointofenslrring the elastic force of the coil spring. and ensuring the strength e of t h e p u l l e y member/the hub s t r u c t u r e , resulting i n a case where a f r i c t i o n a l torque demanded by the user cannot be realized. Then, the invention has been made with a view to solving the problems described above, and an object thereof is to provide a p u l l e y structure t h a t cart suppress t h e application of an excessive force to a coil spring ta thereby prevent the * breakage of the coil spring itself and which can increase the degree of freedom in design by changing freely the frictional torque while r e s t r i c t i n g the frictional torque to vary. MEANS FOR SOLVING THE PROBLEMS According to a first aspect of the invention, there is provided a pulley structure including a cylindrical pulley member around which a belt is l a i d t o extend, a hub structure t h a t is provided inside the pulley member so as to rotate relative to the pulley member, a c o i l spring t h a t is fixed to the hub structure or the pulley member a t one end thereof, a tapering t o which the other end of t h e coil spring is fixed and which has a conical round surface as of a cone whose axis is made up of a r o t a t i o n a l axis of t h e hub s t r u c t u r e , and a f r i c t i o n a l member t h a t is inserted t o be interposed between t h e conical round surface o f t h e t a p e r i n g and the pulleymember 0 or between the conical round surface of the tapering and the hub s t r u c t u r e , wherein the coil spring is insefted t o be interposed in place while being compressed i n the direction of t h e r o t a t i o n a l axis of the hub s t r u c t u r e , and the tapering, the f r i c t i o n a l member and the pulley member are brought i n t o press contact with each other, or t h e t a p e r i n g , the frictional member and the hub structure are brought into press contact with each other by v i r t u e of a restoring force of t h e coil * spring, According t o the configuration described above, the taperingispressedforcontsctbyvirtueofthe r e s t o r i n g force of the coil spring, and the conical round surface of the tapering that is so pressed for contact is then brought into press contact with the pulley member or the hub s t r u c t u r e via the f r i c t i o n a l member. By adopting t h i s configuration, a f r i c t i o n a l torque is generated between t h e conical round surface of the tapering and the f r i c t i o n a l member. Then, when an i n p u t torque t h a t is larger than this f r i c t i o n a l torque is inputted fromthepulleymemberorthehubsts~cture,a r e l a t i v e s l i p is made to be generated between the conical round surface of the tapering and t h e f r i c t i o n a l member on a press contact surface therebetween soas toprevent the coilspringfrombeing d i s t o r t e d t o a c e r t a i n l e v e l o r l a r g e r . B y p r e v e n t i n g t h e c o i l spring from being distorted t o the certain level or larger i n @ t h e way described above, the durability of the c o i l spring can be increased. Additionally, the f r i c t i o n a l member and the tapering having t h e conical round surface can be produced with goodaccuracybycuttingormoldinguringamold, a n d t h e r e f o r e , the f r i c t i o n a l torque can be r e s t r i c t e d from varying. In addition, by changing freely the material/performance of the f r i c t i o n a l member, a desired f r i c t i o n a l torque can be set, thereby making it possible to increese the degree of freedom * i n design. Additionally, according t o a second aspect of the invention, there is provided a pulley s t r u c t u r e as set forth i n the first aspect of t h e invention, wherein the frictional member is fixed t o t h e pulley member or the hub s t r u c t u r e , so t h a t the f r i c t i o n a l member is allowed t o s l i p r e l a t i v e t o t h e tapering only. According t o the configuration descxibed above, by fixing t h e f r i c t i o n a l member to the p u l l e y member or the hub structure, t h e f r i c l i o m l member is allowed to s l i p r e l a t i v e t o t h e tapering only.. By adopting this configuration, a value f o r t h e f r i c t i o n a l torque t h a t i s generatedbetweenthe conical round surface of t h e tapering and the f r i c t i o n a l member can be determined only by taking i n t o ~ o r l s i ~ e r a t i oan static f r i c t i o n coefficient value between the f r i c t i o n a l member and @ t h e tapering. In additio.n, a c c a r d i n g t o a t h i r d a s p e c t o f t h e invention, there is provided a pulley .structure as set forth in tne first or second aspect of t h e invention, wherein as a fixing form of the c o i l spring, a form is adapted i n which at least one of one end and the other end of the coil spring is locked on a t least one of the hub structure, the pulley member and the m tapering by v i r t u e of the restoring force of the coil spring w h i l e being e l a s t i c a l l y deformed i n a radial direction, and a s l i p is generated when a r o t a t i o n a l torque is inputted which is l a r g e r than a f r i c t i o n a l torque that is generated between a t least one of the one end and t h e other end of the c o i l spring and a t least one of the hub s t r u c t u r e , t h e pulley member and the tapering. According to the configuration described above, it is possible to generate the frictional torque that is generated between the conical round surface of the tapering and the frictional member and the frictional torque t h a t is- generated between at least one of the ane end and the other end of the coil spring and at least one of t h e hub s t r u c t u r e , the pulley member and the tapering. By adopting t h i s c o n f i g u r a t i o n , the value of t h e frictional torque that is generated between the coil spring and at least one of t h e hub s t r u c t u r e , the pulley @ member and the tapering and the value of the frictional torque that is generated between the conical round surface of the tapering and the frictional member can be changed freely 30 as t o increase the degree of freedom in design. Additionally, according to a fourth aspect of the invention, there is provided a pulley structure as set forth in the third aspect of the invention, wherein the f r i c t i o n a l a torque that is generated between the conical round surface of the tapering and the frictional member and the frictional torque that is generated between the coil spring and at l e a s t one of the hub structure, the pulley member and the tapering have different values. According to the configuration described above, by causing the frictional torque that is generated between the c o n i c a l r o u n d s u x f a c e o f t h e t a p r i n q a n d t h e friction;llmember and t h e frictional torque t h a t is generated between the c o i l spring and a t least one oi the hub structure, t h e pulley member and t h e tapering t o have the d i f f e r s n t values, it is possible to determine basedonthernagnitudeofa r o t a t i o n a l t o r q y e t h a t is inputted from the pulley member or the hub structure where t o cause a s l i p t o occur, t h a t is, either between the conical round surface of the tapering and the f r i c t i o n a l member or between the coil spring and at least one of the hub structure, @ t h e p u l l e y member and the tapering. I t is possible t o provide t h e pulley structure that can suppress the a p p l i c a t i o n of an excess-ive fofce to t h e coil springtotherebypreventthebreakageoftheeoilspsingitself and which can increase the degree af freedom in desi.gn by 0 changing freely the frictional tasq.ue- while r e . s t r i c t i n g the frictional t o r q u e to vary. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram of an auxiliary @ driving belt system of a first embodiment. Fig. 2 is a plan view of a drive p u l l e y s t r u c t u r e according to the first embodiment. Fig. 3 is a sectional view taken along the l i n e A-A that contains a rotational axis of the drive pulley structure shown in Fig. 2 . Fig. 4 is a diagram showing i n detail the drive pulley structure shown in Fig. 3. Fig. 5 i s a p l ~ n v i e w o f a d r i v e p u l l e y s t r u c t u r e a c c o r d i n g to a second embodiment. Fig. 6 is a sectional view taken along t h e line B-B t h a t @ contains a r o t a t i o n a l a x i s o f t h e drive pulley s t r u c t u r e shown i n Fig. 5 . Fig. 7 is a diagram showing i n detail the drive pulley structure shown i n Fig. 6, Fig. 8 i s a s e c t i o n a l view that contains a rotational a x i s of a drive pulley structure according to a t h i r d embodiment. MODES FOR CARRYING OUT THE IHVEZUTION ( First Embodiment) A first embodiment of the invention w i l l be described. As shown in Fig. 1, t h i s embodiment describes an example where the invention is applied to a drive pulley structure 1 that is used in an auxiliary driving belt system 100 t h a t drives an auxiliary (a water pump or an a l t c r n a , t o r ) by torque of an output: s h a f t 1 0 1 o f a m ~ t o r v e h i c l e e n g i n e . Notethat t h e d r i v e pulley structure 1 is used to suppress a variation i n tension of a transmission belt 106 tshat is a t t r i b u t e d t o a change i n rotation of the engine. (Auxiliary Driving b e l t system LOO) Fig. 1 is a schematic block diagram of the auxiliary driving belt system 100 of this embodiment. As is shown in Fig. 1, the auxiliary driving belt system 100 has t h e drive • p u l l e y s t r u c t u r e 1 (a pulley structure) t h a t is connected to I t h e output shaft 101 of the engine (a crankshaft of a r e c i p s o c a t i n g e n g i n e o r a n e c c e n t r i c s h a f t o f a r o t a r y e n g i n e ) , driven shafts (auxiliary shafts) 102, 103 that are connected ro auxiliaries suchas a water pumpandanalterfiator, a d r i v e n pulley structqre 104 that is mounted on the driven s h a f t 102, a driven pulley structure 107 t h a t i s mounted on the driven shaft 103 and t h e transmission belt 106 t h a t is looped over e the drive p u l l e y structure 1, the driven pulley s t r u c t u r e 104, and the driven pulley structure 107. In t h i s embodiment, a V ribbed belt having a p l u r a l i t y of V-shaped ribs t h a t extend p a r a l l e l to each other along a lofigitudihal direction of the belt is used as the transmission belt 106. In the a u x i l i a r y drivi-ng belt system 100, when t.he drive pulley s t r u c t u r e 1 is driven to rotate by the torque of the. output s h a f t 101, t h e tra.nsrnis.sion belt 106 is driven by v i r t u e of t h e rotation of the drive pulley structure 1. Then, the driven p u l l e y structure 184 and the driven pulley structure 107 are driven to r o t a t e as the transmission belt 106- runs in loop, whereby although not shown, the auxiliaries such as the water pump and the a l t e r n a t o r t h a t are connected t o the driven shafts 102, 103 are driven. (Canfiguration of Drive Pulley Structure 1) Next, t h e drive pulley structure 1 that is driven to rotate by virtue of the torme of t h e output shaft 101 will be described in detail. Fig. 2 is a plan view of the drive pulley s t r u c t u r e 1. Additionally, Fig. 3 is a sectional view taken along the line A-A which contains a rotation axis J of the drive pulley structure 1 shown in Fig. 2. In addition, Fig. 4 is adiagram-shawingindetailthedrivepulleystructuxe 1 shown in Fig. 3. As shown in Figs. 2 and 3, the drive pulley s t r u c t u r e 1 has a cylindrical p u l l e y member 2 around which the transmissionbelt106 is laid to extend, a hub structure 3 that is connected to the output shaft 101 and which is provided icside the pulley member 2, a coil spring 4 that is fixed to t h e hub s t r u c t u r e 3 at one end 4a t h e r e ~ f ,a tapering 5 to which t h e other end 4b ~f t h e coil spring 4 is fixed and which has an outer circumferential surface $9 having a conical round surface as of part of a cone whose a x i s is made up of the r o t a t i o n a l axis J of the hub s t r u c t u r e 3, and a frictional member 6 t h a t is inserted to be interposed between the outer circumfe,rential. surface 5a of the tapering 5 that has the conical round surface .as of the cone and the pulley member 2. Additionally, t h e pulley member 2 and the hub structure 3 are connected so as to r o t a t e relative to each other via a rolling bearing 9. Further, a slide bearing 8 is provided so as to be interposed between the hub structure 3 and the tapering 5. The pulley member 2 has a cylindrical, shape, and a plurality of pulley grooves 2a are farmed in an outer circumference of the pulley member 2 so as to extend in a circumferential direction thereof. Then, the transmission belt 106 is l a i d to extend around the outer circumference of the pulley member 2 in such a state that the p l u r a l i t y of @ V-shaped ribs t h a t are provided on an inner circumfexence of the transmission b e l t 106 are in engagement with t h e corresponding pulley grooves 2a. The hub structure 3 has also a cylindrical shape. The output shaft 101 is f i t t e d in a cylindrical i n t e r i o r poftion 3a of the hub structure 3 so as t o extend therethraugh. The output s h a f t 1 0 1 and the hub s t r u c t u r e 3 are connected together by appropriate connecting devices such as b o l t s so as not to rotate relative to each other. Additionally, as materials of whith the pulley member 2 and the hub structure 3 are made, a non-magnetic material (a paramagnetic material, a diamagnetic material or an anti-ferromagnetic material) is raised for each of them. Specifically, as a non-magnetic material, far example, an aluminum alloy, a titanium alloy or a s y n t h e t i c resin is raised. Note t h a t the pulley member 2 and the hbb structure 3 are connected t ~ g e t h e sr o as t o rotate * relative to each other via the rolling bearing 9. As shown i n Fig. 3, the tapering 5 has a substantially U-shaped in section. An inner circumferential surface of t h e tapering 5 has a cylindrical shape, and an o u t e r circumferential surface 5a thereof has t h e cohical round surface as of part of the cane whose aeis is made up of the rotational axis J of the hub structure 3. Here, as shown in @ Fig. 4, when seen in section, an angle 8 that is farmed by an extension 5L of the outer circumferential surface 5a that is formed into the conical round surface and the rotational axis J is set to be in the range of lo or larger to less than 90'. The slide bearing 8 is interposed between the hub structure 3 and the tapering 5, and the hub structure 3 and the tapering 5 are allowed to rotate relative to each other. The frictional member 6 is inserted to be interposed betwaentheoutercircumferenti~lsurface5ahavingtheconical roundsurfaceandthepulleymember2, andthe frictionalmember 6 itself is fixed t o the p u l l e y memher 2 so as not to r o t a t e . It is preferable that the f r i c t i o n a l member 6 is made of a material which is superior in wear resistance and resistance to compression deformation. For example, metals such as brass, plated brass, bronze and p l a t e d bronze and s y n t h e t i c resins such as polyamide, polyacetal and polyarylate are raised. * Additionally, a static f r i c t i o n c o e f f i c i e n t between the tapering 5 and the frictional member 6 i s set to a value that enables a relative slip to be generated between the tapering 5 and the f r i c t i o n a l member 6 when a desired input torque is received. Specifically, the static friction c o e f f i c i e n t is set based on selected m a t e r i a l s for the f r i c t i o n a l member 6 and the tapering 5, a mode of a surface f i n i s h / c o n f i g u r a t i o n (for example, a configuration in which recesses and projections are arranged) of t h e f r i c t i o n a l member 6 t h a t is brought into abutment with the tapering 5 o r the angle 0 formed by the extension 5L of the outer circumferential surface 5a and the rotational axis 3. Used for the coil spring 4 is an angular coil spring i n which an elongated l i n e a r member having a substantially rectangular cross section as shown in Fig. 3 i s formed i n t o a spiral shape. Additionally, the coil spxing 4 is fixed t o the hub structure 3 at the one end 4a thereof and is fixed to t h e tapering 5 at the other end 4b i n such a state that the coil spring 4 is compressed in the direction of t h e rotational a x i s J. Then, the tapering 5 and the f r i c t i o n a l member 6 are brought i n t o press contact with each othex by virtue of a restoring force P of the coil spring 4 so compressed. Specifically, as shown i n Fig. 4, the tapering 5 is pressed for contact by virtue of the restoring fofce P of the coil spring. Then, the outer circumferential surface 5a having the conical round surface of the tapering 5 that is so pressed for cohtact is brought into press contact with the frictional member 6 so as to press it by a vertical drag N t h a t is expressed by t h e following expxession (1) in relation to the angle 8 that is formed by the extension 5L of the outer circumferential surface 5a and the r o t a t i o n a l axis J when seen In addition, a frictional torque Tf that is generated between the outey circtunfekential surface 5a of the tapering 5 and the frictional member 6 when the outer circumferential surface 5a of the tapering 5 is brought i n t o press contact with t h e frictional member 6 so as t o press, it by t h e vertical dzag N expressed by the expression (1) aboye is expressed by the following expression ( 2 ) . ~f=pxNxr=pxP/sinexr . . . ( 2 ) where, p: S t a t i c f r i c t i o n coefficient between t h e tapering 5 and the f r i c t i o n a l member 6 r: Average r a d i u s of t h e o u t e r circumferential s u r f a c e 5a having the conical round surface. (Functiqn of Drive Pulley S t r u c t u r e 1) Next, the function af the drive p u l l e y s t r u c t u r e 1 of t h i s embodiment w i l l be described. Here, t h e function of t h e drive pulley s t r u c t u r e 1 will be described based on an assumption that the output s h a f t 101 rotates when the engine is startedto totate, whereby torque is inputtedintothe drive pulley structure 1 from a hub sttuctuxe 3 side thereof v i a the output shaft 101. Assuming that the torque that is inputted a from t h e hub s t r u c t u r e 3 side via the output shaft 101 is an input torque T (a variable), when t h e r e l a t i o n between T and Tf is something like a relation expressed by the following expression ( 3 ) , there occurs no s l i p on an abutment surface between the outer circumf erenkial surfaced 5a of t h e tapering 5 a n d t h e f r i c t i o n a l m e m b e r 6 , a n d t h e c o i l spxing4 isdistorted in a circumferential direction t~ thereby absorb t h e input torque T. TTf) expressed by the expression (4) described above, the coil spring 204 is not distorted in the circumferential direction, but a s l i p occurs on the abutment surface between the inner circumferential surface 20Sa of the tapering 205 and the f r i c t i o n a l member 206, whe.reby the input torque T is absorbed by the s l i p . According to the configuration described above, the tapering 205 i s pressed for contact by the restoring force P of the coil spring 204, and the inner circumferential surface 205a having the conical raund surface of the tapering 205 that is so pressed for contact i s brought into press contact with the hub structure 203 via the frictional member 206 t o press it. By doing so, the f r i c t i o p a l t o r q u e T f is generatsdbetwcen the inner circumfersntialsurface205ahavingtheconicalround surfaceofthetapering205andthefrictlonalmember206. Then, when the input torque T t h a t is larger than the f r i c t i o n a l torque Tf is inputted from the hub s t r u c t u r e 203, a relative slip is caused to occur on the surface where the inner circumferential surface 205a havingthe conical round surface of the tapering 205 and the frictional member 206 are pressed for contact with each other so that the coil spring 204 can b e p r e ~ e n t e d f r o m b e i n g d i s t o r t e d t o a c e r t a i n l e v e l o r l a r g e r . By preventing t h e c o i l spring 204 from being distorted to the certain level or largeria thisway, thedurabilityofthe coil spring 204 can be increased. Additionally, the frictional membez206andthetapering205havingtheconicalroundsurface that generate the frictional torque Tf can be produced with goodaccuracybycuttingormoldingusingamold, andtherefore, the frictional torque Tf can be r e s t r i c t e d from varying. In addition, by changing f r e e l y thematerial/performance of the f r i c t i o n a l member 206, a desired f r i c t i o n a l torque Tf can be set, thereby making it possible to increase the degree of freedom i n design. Addit ionally, according t o the configuration described above, by fixingthe f r i c t i o n a l m e m b e r 2 0 6 t o t h e h u b s t s u c t u r e 203 so as not to rotate, the f r i c t i o n a l member 206 is allowed t o s l i p relative t o t h e tapering205 only. By doing so, a value f o r t h e frictionaltorqueTfthatisgeneratedbetweentheinner circumferential surface 205a havingthe conical round surface of the tapering 205 and the f r i c t i o n a l member 206 can be determined onlybytaking into consideration a s t a t i c friction coefficient value between the frictional member 206 and the 0 tapefing 205. (Third Embodiment) Next, a driven p u l l e y s t r u c t u r e 301 (a p u l l e y structure) according t o a third embodiment w i l l be described mainly with respect t o different configurations from the first embodiment by omittingthe description of similar c o n f i g u r a t i o n s t o t h o s e of the first embodiment. As shown i n Fig. 8, the driven pulley structure 301 according t o the third embodiment adopts a configuration i n which a spring clutch construction is incorporated in a portion where the mil spring 4 of the drive p u l l e y s t r u c t u r e 1 is provided in t h e first embodifieult. Additionally, the t h i r d embodiment w i l l be described as the driven pulley Structure 301 according tq t h e invention beinq mounted on the driven shaft 102 of the a u x i l i a r y driving belt system ID0 sho,wil in Fig. 1 f o r bse as- a driven pulley structure. Fig. 8 is a sectiona.1 view t h a t c ~ n t a i n sa r o t a t i o n a l a x i s 3 (r of the driven p u l l e y sttucture 301. (Configuration of Driven Pulley Structure 301) In the drivenpulleystructure 3 0 1 a c c o r d i n g t o t h e t h i r d embodiment, the configurationadoptedinthe first embodiment is not adopted in which the coil spring 4 of t h e drive pulley structure 1 is fixed d i r e c t l y t o the tapering 5 a t the other end4bthereof, but as showninFig. 8, a c o n $ t r u c t i o n i s adopted 0 in which the other end 304b of a c o i l spring 304 is elastically deformed in a radially outward direction 0, so t h a t the coil spring 304 is locked t o be mounted in a tapering 305 by v i r t u e o f a r e s t o r i n g f o r c e F ( i n a r a d i a l 1 y i n w a r d d i r e c t i o n ) t h e r e o f . On the other hand, one end 304a of the coil spring 304 is fixed t o a huh s t r u c t u r e 303. (Function of Driven Pulley Structure 301) Next, the function uf the driven p u l l e y structure 301 of this embodiment will be described. F i r s t l y , t h e function of t h e driven p u l l e y structure 301 wf 11 be described based on a case where an output shaft 101 r o t a t e s when t h e engine is s t a r t e d t o r o t a t e a n d t o r q u e i s i n p u t t e d i n t o t h e d r i v e n p u l l e y structure301fromapulleymenbex302 sideofthedrivenpulley s t r u c t u r e 301 via the drive pulley s t r u c t u r e 1 and the transmission belt 106. This case is based on an assumption t h a t a rotational torque of the pulley member 302 exceeds a @ r o t a t i o n a l t o r q u e ofthe h u b s t x u c t u r e 303 o f t h e drfvenpulley structure 301 when the engine is s t a r t e d to rotate. As in the case with the first embodiment, assuming that t h e torque that is inputted i n t o the driven pulley s t r u c t u r e 301 from t h e pulley member 302 side is an i n p u t torque T (a varia,ble) and that a frictional torque gener.ated between an outer circumferential surface 305a of the tapering 305 and a e f r i c t i o n a l member 306 is Tf, when the r e l a t i a n between T and T f i s something like the relation (TTf) expressed by the expression ( 4 1 , the coil spring 304 is contracted i n diameter as a result of the c o n t a c t with the c l u t c h surface 3 0 5 ~of the tapering 305 and is kept held to the c l u t c h surface 305c with thepress contactengagingforcerelat'ivetotheclutchsurface 305c increased, no slip occurring. However, a slip occurs on theabutmentsurfacebetweantheoutercircumferentia~ surface 305a of the tapering 305 and the frictional member 306 to ? thereby absorb t h e input torque T. By absorbing the input torque T i n this way, the r o t a t i o n af t h e pulley member 302 is not transmitted to the hub structure 303, and the pulley member 302 r o t a t e s freely. Next, t h e function af the driven pulley structure 301 w i l l be ciescribed based on .a case where. although the rotational torque of the pulley member ,382 i.s dec:reased via a transmission. b e l t 106 when t h e output shaft 101 stops r o t a t i n g as a result of t h e engine. stopping r o t a t i n g , the r o t a t i o n a l torque of the hub s t r u c t u r e 303 exceeds the r o t a t i o n a l torque of the pulley member 302 du.e.to the i n e r t i a producedwhenthe engine rotates-, t h a t is, a case where torque is inputted i n t a t h e driven p u l l e y s t r u c t u r e from a hub :stru.cture 303 side. Assurningthatthetorquethat i s i n p u t t e d i n t o t h e d r i v e n pulley structure 301fromthehub s t r u c t u r e 303 s i d e is an input e torque T (a v a r i a b l e ) , that a frictional torque generated between the outer c i r c u m f e r e ~ t i a l s u r f a c e3 0 5 a o f t h e t a p e r i n g 305 and the f r i c t i o n a l member 306 is Tf and t h a t a frictional torque generated between t h e c o i l spring 304 and t h e clutch surface 305c is Ts, when the relation between the f r i c t i o n a l torques is such t h a t T