referring to fig1 of the drawings , a vehicle engine 1 comprises an oil pan 1 a and exhaust manifold 2 . the exhaust manifold 2 is connected to an exhaust pipe 3 via a converter 20 . a separate converter 21 is interposed midway in the exhaust pipe 3 . a three - way catalyst 8 and hydrocarbon ( hc ) adsorbent 4 a comprising a coating layer of hydrocarbon ( hc ) adsorbent are housed in series with respect to the flow of exhaust in a converter 21 . an hc adsorbent 4 b identical to the hc adsorbent 4 a is housed in the converter 20 . the hc adsorbents 4 a , 4 b comprise a coating of a layer 6 for adsorbing hc on the surface of a vanadium support 5 , this being coated with a three - way catalyst layer 7 as shown in fig4 . the adsorbent layer 6 and three - way catalyst layer 7 are coated for example by the method disclosed in tokkai hei 5 - 57148 . a converter 20 housing the hc adsorbent 4 a is arranged immediately after the exhaust manifold 2 at a high exhaust temperature . a converter 21 housing the hc adsorbent 4 b is disposed midway in the exhaust gas pipe 3 in a substantially horizontal direction pointing in a direction from the engine room to the rear of a vehicle , and situated under an oil pan 1 a of the engine 1 which has good high temperature retention characteristics . the heat capacity of a part 3 a of the exhaust gas pipe 3 situated between the converters 20 , 21 is set so that the temperature of the hc adsorbent 4 b does not exceed an upper limiting temperature at which hc can be adsorbed until discharge of hc by the adsorbent 4 a is complete . as the temperature of the exhaust passage 3 has not increased much before warming up of the engine 1 is complete , the three - way catalyst 8 and three - way catalyst layer 7 of the hc adsorbents 4 a , 4 b are not activated . therefore , in this stage , purification by oxidation of hydrocarbons ( hc ) in the exhaust cannot be expected to occur . however , all the hydrocarbons ( hc ) which have passed through the three - way catalyst 8 are effectively adsorbed by the hc adsorbents 4 a , 4 b . when a certain time has elapsed after engine startup , the temperature of the hc adsorbent 4 a rises as shown by the line a in fig5 and eventually exceeds an hc adsorption upper limiting temperature t 1 . subsequently , the adsorbent layer 6 of the hc adsorbent 4 a stops adsorbing hydrocarbons ( hc ), and discharge of adsorbed hydrocarbons ( hc ) begins . on the other hand , when the temperature of the hc adsorbent 4 a exceeds a catalyst activation temperature t 2 , hydrocarbons discharged by the hc adsorbent 4 a are oxidized by the upper three - way catalyst layer 7 to become water vapor and carbon dioxide . while the temperature of the hc adsorbent 4 a is increasing from t 1 to t 2 , the three - way catalyst layer 7 is not activated , so hydrocarbons ( hc ) discharged from the hc adsorbent 4 a flow into the downstream converter 21 without being oxidized . at that time , if the hc adsorbent 4 b of the converter 21 has not reached the upper limiting temperature t 1 at which adsorption is possible , the hydrocarbons flowing into the converter 21 are adsorbed by the hc adsorbent 4 b . however , if the temperature of the hc adsorbent 4 b exceeds the upper limiting temperature at which adsorption is possible before the hc adsorbent 4 a has reached the catalyst activation temperature t 2 as shown by the line d of fig5 a time zone is created where the hydrocarbons flowing into the converter 21 are discharged without being adsorbed or oxidized as shown by the region x of the figure . to deal with this problem , in this purification device , the temperature rise characteristics of the hc adsorbent 4 b of the converter 22 are set as shown by the line b in fig5 . this is achieved by setting the heat capacity of the part 3 a of the exhaust pipe 3 situated between the converters 20 , 21 as described hereabove . as a result , even when the temperature of the hc adsorbent 4 a of the converter 20 becomes equal to or greater than the upper limiting temperature t 1 and is lower than the catalyst activation temperature t 2 , the hc adsorbent 4 b of the converter 21 has not reached the upper limiting temperature t 1 at which hc adsorption is possible , so the whole amount of hydrocarbons flowing into the converter 21 is adsorbed by the hc adsorbent 4 b . in the converter 21 , the temperature of the hc adsorbent 4 b eventually reaches the upper limiting temperature at which hc adsorption is possible , and discharge of adsorbed hydrocarbons begins . at the stage when the catalyst activation temperature t 2 is exceeded , these hydrocarbons are oxidized by the three - way catalyst 7 . in the converter 21 , as in the case of the converter 20 , during the interval when the temperature of the hc adsorbent 4 b increases from t 1 to t 2 , a time zone is created where hydrocarbons temporarily cannot be processed . however , as this is a two - stage processing , the discharge amount of hydrocarbons from the converter 21 in this time zone is less than the inflow amount of hydrocarbons to the converter 20 . in the hc adsorbents 4 a , 4 b , the adsorbent layer 6 is coated on the surface of the carrier 5 , and the three - way catalyst layer 7 is coated on the absorbent layer 6 . in the temperature region at and above the catalyst activation temperature t 2 , hydrocarbons released from the adsorbent layer 6 which is the underlayer are oxidized by the three - way catalyst layer 7 which is the upper layer , and a good purification efficiency is obtained . according to experiments performed by the inventors , it was found that when the upper limiting temperature t 1 of hc adsorption is 200 ° c . and the catalyst activation temperature t 2 is 270 ° c . in the above device , when the heat capacity of the part 3 a of the exhaust pipe 3 is set to 80 cal /° c ., the temperature of the hc adsorbent 4 b reaches the upper limiting temperature t 1 at which hc can be adsorbed when the hc adsorbent 4 a reached the catalyst activation temperature t 2 . this corresponds to the line c of fig5 . also , in the case of the exhaust pipe 3 of an ordinary vehicle engine , the heat capacity of the part 3 a is 80 cal /° c . when the length of the part 3 a is set to approximately 180 mm . therefore , the effect of this invention is generally achieved by setting the length of the part 3 a to 180 mm or more . next , a second embodiment of the invention will be described referring to fig2 . according to this embodiment , a converter 22 is provided in the exhaust pipe 3 downstream of the converter 21 of the first embodiment . this converter 22 is installed in a part underneath the floor of the vehicle . an hc adsorbent 4 c comprising an adsorbent layer 6 and a catalyst layer 7 similar to those of the hc adsorbents 4 a , 4 b is housed in the converter 22 . a heat capacity q 2 of a part 3 b of the exhaust pipe 3 between the hc adsorbents 4 b , 4 c is set larger than a heat capacity q 1 of the part 3 a between the hc adsorbents 4 a , 4 b . this is accomplished by setting the length of the part 3 b to be longer than that of the part 3 a . however , the heat capacity can be increased without lengthening the part 3 b if a cornice 9 is provided as shown in the figure . the heat capacity q 2 of the part 3 b is set so that the hc adsorbent 4 c does not exceed the upper limiting temperature at which hc is adsorbed before the hc adsorbent 4 b reaches the catalyst activation temperature t 2 , i . e . so that the temperature increase characteristics shown by the line e in fig6 are obtained . according to this embodiment , there is one more hydrocarbon adsorption , discharge and oxidation process than in the aforesaid first embodiment , so a higher hydrocarbon processing performance is obtained . next , a third embodiment of this invention will be described referring to fig3 . in this embodiment , a converter 23 of different internal construction is used instead of the converter 22 of the aforesaid second embodiment . the converter 22 housed only the hc adsorbent 4 c , but the converter 23 comprises a three - way catalyst 10 downstream of the hc adsorbent 4 c , and an hc adsorbent 4 d situated still further downstream . the construction of the three - way catalyst 10 is identical to that of the three - way catalyst 8 , and the construction of the adsorbent 4 d is identical to that of the adsorbent 4 c . the three - way catalyst 10 functions as an exhaust passage between the hc adsorbents 4 c , 4 d . in other words , the temperature increase characteristics of the hc adsorbent 4 d is determined by the heat capacity of the three - way catalyst 10 . the heat capacity of the three - way catalyst 10 is determined so that the temperature of the hc adsorbent 4 d does not exceed the upper limiting temperature t 1 at which hc can be adsorbed before the hc adsorbent 4 c reaches the catalyst activation temperature t 2 . due to this setting , the distance between the hc adsorbents 4 c , 4 d can be set shorter than when the passage is formed by a pipe or cornice . according to this embodiment , there is one more hydrocarbon adsorption , discharge and oxidation process than in the aforesaid second embodiment , so an even higher hydrocarbon processing performance is obtained than in the aforesaid second embodiment . in the aforesaid embodiments , in any of the aforesaid converters , part of the hydrocarbons ( hc ) discharged by the adsorbent layer of the hc adsorbent is oxidized by the three - way catalyst layer and part is discharged from the converter without being oxidized after the temperature of the hc adsorbent has reached the upper limiting temperature t 1 at which hc can be adsorbed . in other words , the chance of oxidizing hydrocarbons ( hc ) increases the larger the number of converters , and the amount of hydrocarbons becomes less each time the exhaust gas passes through a converter . as a result , the volume of hc adsorbent can be reduced further downstream , which also reduces the weight and cost of the exhaust purification device . the contents of tokugan hei 10 - 149899 with a filing date of may 29 , 1998 in japan , and tokugan hei 11 - 22792 with a filing date of jan . 29 , 1999 in japan , are hereby incorporated by reference . although the invention has been described above by reference to certain embodiments of the invention , the invention is not limited to the embodiments described above . modifications and variations of the embodiments described above will occur to those skilled in the art , in light of the above teachings . for example , the volume of hc adsorbents is set to be constant , but the coating amount of hc adsorbent may be reduced further downstream . the embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows :