preferred embodiments of a filler neck cap according to the present invention will now be described with reference to the drawings . fig1 to 3 show a first embodiment of the filler neck cap according to the invention . this filler neck cap is of the type in which a cover grip 2 of an outer cap 3 is engaged with and retained by an inner wall surface 9 of a filler box which surrounds the outer periphery of the cover grip 2 . as clearly understood from fig1 and 2 , the filler neck cap of the first embodiment is designed to be secured in an opening of a filler neck 10 . the filler neck 10 includes a threaded portion in the vicinity of the bottom surface of the filler box . the filler neck cap comprises the outer cap 3 and an inner cap 4 , the outer cap 3 including a rotatable knob 1 and the cover grip 2 . the rotatable knob 1 is associated with the cover grip 2 through a planetary gear mechanism . in the illustrated embodiment of the invention , the cover grip 2 is shown to have a substantially dodecagonal shape in cross section . the cover grip 2 can be held by the inner surface 9 of the filler box wall surrounding the cover grip 2 in conformity with the outer surface thereof . assuming that the cover grip 2 is held by the wall surface 9 from the beginning of the tightening operation of the filler neck cap , the whole operation unfavorably becomes tedious . accordingly , the inner cap 4 having a male screw thread 11 on its outer periphery is firstly inserted into the filler neck 10 at a certain distance by screwing , and then , the cover grip 2 is engaged with the wall surface 9 of the filler box . more particularly , after an o - ring 12 fitted on the inner cap 4 forcedly contacts with an upper end of the filler neck 10 , the cover grip is held by the wall surface of the filler box in order to quickly rotate the cap around the filler neck . referring to fig3 the planetary gear mechanism includes a sun gear 5 and four planet gears 6 . the sun gear 5 is rotatably attached on a center of a carrier disk plate 8 . the carrier disk plate is rotatably connected to an upper face of the inner cap 4 . the planet gears 6 are also rotatably connected on the carrier plate 8 in such a manner that they are located around the sun gear 5 . in the illustrated embodiment , a square pin 13 is integrally formed with the sun gear 5 and projects from an upper face of it . the square pin 13 is connected to the knob 1 , so that the sun gear 5 is moved by the knob 1 . the sun gear and the knob can be formed integrally with each other . if so , the number of steps of assembling the component parts can be decreased and the transmission of movement between the knob and the sun gear can be realized . the planet gears 6 are meshed with a ring gear 7 formed on an inner periphery of the cover grip 2 . in the first embodiment , a ratchet 14 is formed on an outer periphery of the carrier disk plate 8 , and latches 15 are provided on an inner periphery of the inner cap 4 in association with the ratchet 14 . when the carrier disk plate 8 is rotatingly driven by the planet gears 6 which are rotated by the sun gear 5 , the rotation torque of the carrier disk plate is transmitted from the ratchet 14 to the latches 15 , whereby the knob 1 can forcibly drive the inner cap 4 to be screwed into the filler neck 10 . thus , the ratchet 14 and the engaged latch 15 constitute a torque limitation mechanism . alternatively , the ratchet 14 may be provided on the inner periphery of the inner cap 4 and the latches 15 may be provided on the outer periphery of the carrier disk plate 8 . the ratchet 14 includes a hole 17 for giving elasticity in a radial direction of the carrier disk plate 8 to the ratchet . in fig3 the hole 17 is formed in the vicinity of the root portion of the ratchet 14 so that the ratchet 14 can deform in the radial direction of the carrier disk plate 8 when the excessive rotational force is applied to the knob 1 . the ratchet 14 may ordinarily press - contact with the latch 15 to realize favorable engagement between them more securely . according to the planetary gear mechanism in the first embodiment , the force applied by the user is multiplied by four times . a high amplification rate of the torque is thus obtained . the diameters of the sun and planet gears of the planetary gear mechanism are selectively predetermined . when a tightening torque limitation value is set at 80 kgcm , an operating torque of 20 ± 5 kgcm need only be applied . in the filler neck cap according to the first embodiment , when the sun gear 5 is rotated by the knob 1 , the planet gears 6 revolve around the sun gear while a portion of the filler neck cap or the cover grip 2 of the outer cap 3 is securely held , so as to amplify the operating torque . at this time , it is important that the cover grip 2 is prevented from rotation with respect to the rotating knob 1 . fig4 to 6 illustrate a second embodiment of the invention in which another method of holding the cover grip is shown . according to the second embodiment , a lower edge of a cover grip 2 of an outer cap 3 is formed with a notched portion 18 like the teeth of a saw . the notched portion 18 is engaged with protruded portions consisting of three projections 16 formed at an upper edge of a filler neck 10 . in such a state that a filler neck cap is tightly screwed in the filler neck 10 , the cover grip 2 is engaged with the filler neck 10 by means of these projections 16 so as to be restrained from rotation . other parts and structures of the filler neck cap of the second embodiment except the above - described parts are similar to those of the first embodiment , as recognized from comparisons between fig4 and fig1 and between fig5 and fig2 and as seen in fig3 . therefore , the description thereof will be omitted . fig7 and 8 illustrate a third embodiment of the invention . as clearly understood from these figures , the third embodiment differs from the second embodiment in the following three points that : a sun gear 5 and a knob 1 of an outer cap are integrally formed of plastic material ; the number of planet gears 6 to engage with the sun gear 5 is three ; and latches 15 are formed on a carrier disk plate 8 and a ratchet 14 is formed on an inner cap 4 , in contrast to the second embodiment . accordingly , the number of component parts in the third embodiment is smaller than that in the second embodiment , and the assembling work is more facilitated . further , a mechanism for restraining rotation of a cover grip 2 of the outer cap is structured such that six projections 20 at a lower edge of the cover grip 2 are engaged with a metallic protruding piece 21 fixedly attached to an outer wall of a filler neck 10 by spot welding or the like . with the above structure , working processes of machining an end edge of the filler neck pipe are advantageously simplified , as compared with the second embodiment . according to the filler neck cap of the present invention , the operating torque is suitably amplified by making use of the planetary gear mechanism . in the planetary gear mechanism , the gear ratio ( r ) between the sun gear 5 and the ring gear 7 has a relation to the torque amplification rate ( 1 + r ). the planet gears 6 mainly serve to transmit the movement from the outer cap to the inner cap and rotate reversely to the sun gear . for the reasons , one or more planet gears may be used to sufficiently fulfill such a function . moreover , in order to surely and stably carry out the movement transmitting function , it is required to locate the planet gears symmetrically with respect to the sun gear , as seen in the first to third embodiments . according to the filler neck cap of the invention , the o - ring is pressed against the filler neck with the tightening torque obtained by amplifying the operating torque , so that high airtightness at the filler neck can be realized , which conforms to the regulation for controlling an hc ( hydrocarbon ) transpiration . moreover , the filler neck cap of the invention can be operated as readily as the conventional one , and it can be used for the existing automobiles . the filler neck cap may be made more compact if necessary , so that a cost saving effect can be increased and utility of the filler neck cap can be improved .