fig1 to 3 show one embodiment of the present invention . a shock absorbing type steering shaft 11 of the present invention is corresponding to the shaft 8 in fig6 . this steering shaft is formed , as shown in fig1 by bringing a female serration 15 and a male serration 17 formed , respectively , on the peripheral surfaces of shafts 12 and 13 into engagement with each other at a fore end portion of the outer shaft 12 ( the left end portion as viewed in fig1 ) which is one end portion of the outer shaft 12 and a rear end portion of the inner shaft 13 ( the right end portion as viewed in fig1 ) which is one end portion of the inner shaft 13 . in order to prepare such shock absorbing type steering shaft 11 of the present invention , in the same manner as that of the prior invention shown in fig1 and 20 , the outer peripheral surface of the outer shaft 12 is pressed inwardly in the diametrical direction thereof by a pair of pressing pieces 20 a ′ and 20 a ′, which are displaced toward each other , in the state in which these both serrations 15 and 17 are engaged with each other . then , the fore end portion of the outer shaft 12 and the rear end portion of the inner shaft 13 are pressed in the radial direction so that the cross sections of these both end portions are plastically deformed into the respective elliptical shapes , as shown in fig1 . subsequently , the outer shaft 12 and the inner shaft 13 are pressed toward each other so that the fore end portion of the outer shaft 12 and the rear end portion of the inner shaft 13 are laid to overlap each other only by a desired length l in the axial direction , as shown in fig1 . thus the both shafts 12 and 13 are tightly fitted to each other to be brought into pressure - fitted engagement at two locations that are apart from each other in the axial direction and into loose engagement at any part between these locations , as shown in fig7 . at a secondary collision , as shown in fig2 , a fitting length or overlapped length between the outer shaft 12 and the inner shaft 13 is larger than the above - mentioned normal fitted length l so that the total length of the shock absorbing type steering shaft is contracted . in case of the shock absorbing type steering shaft 11 in the present embodiment , a film 24 made of solid lubricant ( see fig3 ) is formed along the total length l 17 ( l 17 & gt ; the desired length l ) of the male serration 17 on the surface of the male serration 17 which is formed on one end portion of the inner shaft 13 . the material and the thickness of the solid lubricant for forming this film 24 are selected in terms of the designing in accordance with a desired contraction load and bending rigidity . the material to be used must have excellent sealing performance onto the surface of the male serration 17 . as such material , a molybdenum disulfide ( mos2 ) type lubricant which is available in market as a trade name “ drilube ” manufactured by toyo drilube co ., inc ., or fluoroplastic such as ptfe , fep may be used preferably . it is required to secure a certain thickness of the film to prevent a friction between the metals for forming the outer shaft 12 and the inner shaft 13 respectively ( normally steel or aluminum alloy ). however , if the thickness of the film 24 , which itself has a low rigidity , is too large , the bending rigidity between the outer shaft 12 and the inner shaft 13 which have the end portions fitted to each other with this film 24 therebetween becomes low . in considering these circumstances , the thickness of the film 24 is preferably regulated into a range from 0 . 005 to 0 . 015 mm when “ drilube ” is used as the solid lubricant . in case of the shock absorbing type steering shaft 11 constituted as described above , the female serration 15 which is formed on the inner peripheral surface at one end of the outer shaft 12 is brought into slidable contact with the male serration 17 which is formed on the outer peripheral surface at the end of the inner shaft 13 with the film 24 of solid lubricant therebetween . there is no chance , or little chance , if any , that the metals ( steel or aluminum alloy ) which respectively constitute the outer shaft 12 and the inner shaft 13 are brought into direct contact with each other . consequently , the slidable contact state between the female serration 15 and the male serration 17 becomes stable and the contraction load of the shock absorbing type steering shaft 11 becomes stable . as a result , it is possible to design more easily a steering apparatus for an automobile which is arranged to include this shock absorbing type steering shaft 11 . particularly , when one of the outer shaft 12 and the inner shaft 13 is made of steel and the other is made of aluminum alloy to reduce the weight of the steering shaft , damages such as plucking or scoring may be easily brought about in this condition on the sliding surface between female serration 15 and the male serration 17 . however , according to the present invention , these damages can be prevented even in such case so that the shock absorbing type steering shaft 11 can be contracted in a stable manner . results of experiments conducted to confirm the effect of the present invention will be described below . in the experiments , a shock absorbing type steering shaft 11 as shown in fig1 and 21 is prepared by the method according to the prior invention described with reference to fig1 to 21 to detect an influence of the presence or the absence of the film 24 which is made of solid lubricant on the surface of the male serration 17 on a contraction load of the shock absorbing type steering shaft . both the outer shaft 12 and the inner shaft 13 are made of steel , and a targeted value for a load pp for pressing the outer shaft 12 and the inner shaft 13 by means of the pair of pressing pieces 20 a ′ and 20 ′ ( fig2 ) is set at 3300 kgf . since this pressing load unavoidably fluctuates to some extent , the load values that are actually applied are shown in table 1 . the desired length l by which the fore end portion of the outer shaft 12 and the rear end portion of the inner shaft 13 are laid to overlap each other in the axial direction is set at 60 mm . the conditions for the experiments are set to be the same as much as possible in all experiments except the presence or the absence of the film 24 . note that s - 6150 ( trade name ) that is a fluoroplastic type “ drilube ” is used as the solid lubricant for forming this film 24 . results of the experiments conducted under such conditions are shown in tables 1 and 2 and fig4 . in tables 1 and 2 , the maximum load ps for pressing indicates the maximum value for the load which is required when the outer shaft 12 and the inner shaft 13 respectively forming the first and second deformed portions 18 and 19 are brought toward each other to constitute the shock absorbing type steering shaft 11 , as shown in fig1 , and corresponds to the peak value shown in the left end portion of fig4 . the contraction load p 1 in the “ outer + inner ” column indicates a load which is required for contracting the shock absorbing type steering shaft 11 in the state in which both the first and second deformed portions 18 and 19 are in the pressure - fitted state ( the range l 1 in fig4 ), as shown in fig1 and 21 . further , the contraction load p 2 of the “ inner only ” column indicates a load which is required for contracting the shock absorbing type steering shaft 11 in the state in which the first deformed portion 18 is taken out from the pressure - fitted state and only the second deformed portion 19 and the inner shaft 13 are pressure - fitted to each other ( the range l 2 in fig4 ), as shown in fig2 . in fig4 the solid line a indicates a relation between a contraction load and an amount of contraction ( stroke ) in case the film 24 is not provided , and the broken line b indicates a relation between a contraction load and an amount of contraction in case the film is provided , represented by the respective average values thereof . as clearly seen from tables 1 and 2 and fig4 showing the results of the experiments , according to the present invention , it is possible to obtain a reduced and stabilized contraction load for the shock absorbing type steering shaft 11 . the point of this invention lies in that the contraction load can be stabilized . the magnitude of the contraction load can be freely adjusted by varying a load for pressing the outer shaft 12 and the inner shaft 13 by means of the pair of pressing pieces 20 a ′ and 20 a ′. more specifically , if this load for pressing is increased , the contraction load can be increased . on the other hand , if this pressing load is decreased , the contraction load can be decreased . accordingly , if the contraction load can be stabilized as mentioned above , the contraction load of the shock absorbing type steering shaft 11 can be set near the targeted value . that is , for designing a steering apparatus for an automobile capable of satisfactorily protecting the driver , a relation between a contraction load and an amount of contraction of a shock absorbing type steering shaft 11 to be incorporated in this steering apparatus is regulated as indicated by the solid line α in fig5 . however , since it is difficult to regulate this relation exactly as indicated as the solid line α , the relation is regulated into a range between two broken lines β , β in the same figure . the narrower the gap between these two broken lines β , β is , the smaller a fluctuation in the relation between the contraction load and the amount of contraction of the shock absorbing type steering shaft 11 becomes . thus , it becomes easier to design the steering apparatus for an automobile . according to the present invention , it is possible to reduce the fluctuation in the relation between the contraction load and the amount of contraction of the shock absorbing type steering shaft 11 by reducing the gap between the two broken lines β , β . note that if the film 24 of solid lubricant is formed on the surface of the female serration 15 , instead of the surface of the male serration 17 , or on the surfaces of both the male and female serrations 17 and 15 , the same operation and effect can be obtained . when the film 24 is formed on the surfaces of both the male and female serrations 17 and 15 , the total thickness of the film 24 is set within the above range . when one or both of the outer shaft 12 and the inner shaft 13 are made of nonferrous metals such as aluminum alloy , the film 24 is formed on the surface of the serration which is formed on the nonferrous metal shaft , or on the surfaces of the both serrations . since constituted and operated as described above , the shock absorbing type steering apparatus of the present invention contributes to realize a steering apparatus which can effectively protect the driver at a secondary collision . in addition , since the film made of solid lubricant may be thin , the bending rigidity of the connection between the outer and inner shafts is not reduced to cause a substantial problem for practical use . further , even when the shock absorbing type steering apparatus is formed by combining steel and aluminum alloy as in the above examples , it is possible to prevent damages such as plucking or scoring to contract the shock absorbing type steering shaft in a stable manner .