referring to fig1 a front derailleur of the invention is mounted on a seat tube s of a bicycle tightening band base member 1 . the base member 1 , as shown in fig2 comprises a pair of half - circular bands 2 and 3 which are pivotally connected at their one ends through a pivot pin 4 and are provided at their other ends with noses 5 and 6 . the noses 5 and 6 are screwably tightened by a bolt 7 and a nut 8 , thereby fixing base member 1 to seat tube s . also , the base member 1 has at one half - circular band 2 a mount 9 extending radially outwardly and horizontally with respect to seat tube s . a pair of first and second linkage members 10 and 11 are disposed below the mount 9 and extend horizontally and longitudinally of the bicycle with respect to seat tube s . the linkage members 10 and 11 are pivoted at the front ends thereof swingably to mount 9 through first and second pivot shafts 12 and 13 having substantially vertical axes , and pivotally carry at the rear ends a chain guide 14 through third and fourth pivot shafts 15 and 16 having substantially vertical axes respectively . when base member 1 is fixed to seat tube s , a lengthwise center line of the respective linkage members 10 and 11 becomes horizontal as shown in fig1 . as a result , linkage members 10 and 11 are smaller in their projected areas longitudinal of the bicycle than conventional linkage members which are vertically arranged . the first and second linkage members 10 and 11 , as shown in fig2 are disposed in parallel to each other and inside and outside with respect to seat tube s . the first linkage member 10 disposed inside is provided at the front end with stoppers 10a and 10b as shown in fig4 . the second linkage member 11 disposed outside is provided at the front end with an arm 11a extending frontward from the pivot point of pivot shaft 13 as shown in fig4 the arm 11a carrying a fixture 17 for a control wire w . the mount 9 has , at a side supporting first linkage member 10 , a recess 9a receiving therein stoppers 10a and 10b and , at the laterally outer surface , two adjusters 18 and 19 to contact with stoppers 10a and 10b corresponding to swinging motion of respective linkage members 10 and 11 , the adjusters 18 and 19 controlling a range of swinging motion of first and second linkage members 10 and 11 . the band 2 having mount 9 , as shown in fig3 is provided with an extension 2a which extends downwardly and has a wire guide 20 having a guide groove 21 for guiding control wire w to fixture 17 . the guide groove 21 , as shown in fig2 and 3 , is open upwardly and provided at the lower end with a stop 22 for securing an outer sheath o guiding control wire w . the wire w extends from a control lever ( not shown ), is guided by outer sheath o , passes through guide groove 21 at wire guide 20 to reach fixture 17 , and is fixed thereto . the wire w , when pulled , forcibly swings linkage members 10 and 11 and , when loosened , elastically returns them through a return spring to be hereinafter described . the chain guide 14 has a pair of opposite guide plates 23 and 24 and is supported to linkage members 10 and 11 through a mounting member 25 . the mounting member 25 , as shown in fig4 is provided with a pair of mounting noses 26 and 27 projecting inwardly with respect to seat tube s and a pair of mounting noses 28 and 29 projecting outwardly with respect to the same , the mounting noses 26 , 27 , 28 and 29 each having a shaft bore , so that the rear end of linkage member 10 is inserted between noses 26 and 27 , and that of linkage member 11 between the noses 28 and 29 , both the rear ends being supported by third and fourth pivot shafts 15 and 16 respectively . the chain guide 14 is fixed to mounting member 25 by pins 30 and 31 . a return spring 32 is interposed between the chain guide 14 and the first linkage member 10 and biases chain guide 14 to be kept stationary at a position corresponding to the smaller diameter chain gear , i . e ., at the so - called low normal condition , the chain guide 14 moving in the direction of the arrow f in fig2 to switch the chain from the smaller diameter chain gear to the larger diameter chain gear . when the front derailleur of the invention constructed as just described is fixed to seat tube s , the linkage members 10 and 11 are disposed horizontally and directed longitudinally of the bicycle with respect to seat tube s . hence , the smaller projected areas of linkage members 10 and 11 longitudinal of the bicycle can considerably reduce an air resistance against linkage members 10 and 11 and base member 1 . when the chain is switched from the smaller diameter chain gear to the larger diameter one , the control lever is operated to pull control wire w to swing the second linkage member 11 together with the first linkage member 10 counterclockwise in fig2 so that chain guide 14 shifts in the direction of the chain movement ( the direction of the arrow f in fig2 ) to reach the position corresponding to the larger diameter chain gear . consequently the chain is not forcibly pressed against the inside surface of the larger diameter gear , thereby smoothly engaging with the teeth thereof . when the chain is reswitched to the smaller diameter gear , control wire w is loosened and return spring 32 acts to return chain guide 14 to the position corresponding to the smaller diameter chain gear . in this instance , the chain is smoothly switched to the smaller diameter chain gear in the same manner as a conventional front derailleur . the outer sheath o used in the aforesaid embodiment is not inevitably necessary . in the case where no outer sheath o is used , wire w is pulled against return spring 32 or loosened , and linkage members 10 and 11 swing to reciprocate chain guide 14 axially of the chain gears to switch the chain to a desired chain gear . in this instance , wire w , when loosened , shakes outwardly with respect to guide groove 21 and may escape therefrom . a remedy for the above is that guide groove 21 , as shown in fig5 through 7 , has a depth d ( fig8 ) in the shaking direction of the loosened wire w which covers a shaking width of wire w , and a barrier wall 40 is provided at the open side of guide groove 21 , i . e ., at the wire w outwardly shaking side , covering groove 21 , so that between the barrier wall 40 and the upper edge of wire guide 20 is formed a gap d of width smaller than a diameter of wire w and guide groove 21 is open substantially perpendicularly to the shaking direction of wire w . in the above discussion of the shaking direction and shaking width of wire w it is meant the direction and width of shaking wire w when loosened by the control lever operation . since the wire guide 20 is constructed as the above , wire w , even when shaking within guide groove 21 , is not hindered from moving by operation of the control lever and , even when shaking excessively , is prevented by barrier wall 40 from escaping from guide groove 21 . the embodiment shown in fig5 to 7 uses no outer sheath o but only the control wire w and is provided with barrier wall 40 integral with base member 1 as shown in fig8 . the barrier wall 40 may alternatively be formed at an insert 41 made from synthetic resin and separate from base member 1 as shown in fig9 the insert 41 being inserted in guide groove 21 . in this instance , the insert 41 is u - like shaped in section to cover the bottom of guide groove 21 , thus guiding the wire w more smoothly . alternatively , an insert 41 of synthetic resin like that shown in fig9 may be interposed within guide groove 21 shown in fig8 . or , the insert 41 , as shown in fig1 , may be formed in an upwardly open u - like shape in section and inserted in guide groove 21 to thereby guide the wire w through the bottom of insert 41 . the above inserts 41 each are provided with a projection 42 having a slit , and the base member 1 has a stepped through bore 43 , the projection 42 being inserted into through bore 43 to fix insert 41 . in the above described embodiment , a free end of wire w secured to fixture 17 projects from the second linkage member 11 and is dangerous to the cyclist . such danger can be eliminated by the provision of a retaining means which retains the free end to the second linkage member 11 . such a retaining means is shown in fig6 and comprises a wire groove 51 formed on the upper surface of the second linkage member 11 and directed from the outside thereof with respect to seat tube 1 to the inside as shown in fig1 and 12 . the retaining means in fig6 comprises a first wire guide groove 52 and a second wire guide groove 53 as shown in fig1 and 12 , the first wire guide groove 52 guiding wire w to fixture 17 , the second wire guide groove 53 guiding the projecting free end of wire w from fixture 17 to wire groove 51 , so that wire w is guided at its end into wire groove 51 and retained to the wall thereof by means of the elasticity of wire w . it is preferred that wire groove 51 , as shown in fig1 , extend toward the fourth pivot shaft 16 which pivots chain guide 14 , that the pivot shaft 16 have a head 16a which partially overlaps with wire groove 51 , and that the free end of wire w retained to the wall of wire groove 51 is inserted below the head 16a , thereby preventing the free end of wire w from rising . in the above construction , the free end of wire w secured to fixture 17 is bent at an end of the second wire guide groove 53 and given an elastically restoring force and then guided into wire groove 51 , thereby being retained at the wall of groove 51 and held by head 16a . hence , the free end of wire w is surely retained to linkage member 11 from outside thereof to inside thereby avoiding a careless escape or external exposure when in use with the front derailleur . in the embodiment shown in fig5 to 7 , chain guide 14 , intead of using mounting member 25 , is provided at the inner guide plate 24 with mounting noses 26 and 27 , so that the first linkage member 10 is inserted between the mounting noses 26 and 27 and supported by the third pivot shaft 15 . a connector 33 bridges guide plates 23 and 24 and is supported to the second linkage member 11 by the fourth pivot shaft 16 . alternatively , the retaining means may comprise a through bore 54 provided at linkage member 11 , as shown in fig1 , or a projection 55 of an inverted l - like shape provided at the same , as shown in fig1 and 17 . the projection 55 may be molded integrally with linkage member 11 or cut and raised from a metallic plate formed in linkage member 11 . while preferred embodiments of the invention have been described using specific terms , such description is for illustrative purpose only , and it is to be understood that changes and variations may be made without departing from the spirit or scope of the invention which is defined in the following claims .