the construction of the present invention will be described in detail hereinafter by referring to embodiments shown in the accompanying drawings . a motor 7 is fixed to a base plate 6 of an automatic drafting machine ( fig1 ). a disk like stocker holder 9 is affixed to the output shaft 8 of the motor 7 . a fitting protrusion 9a protrudes from the stocker holder 9 . a plurality of known pen holding portions ( see fig6 ) are formed along a circumferential direction of the upper disk like part of a penstocker main body 10 . these pen holding portions hold pens 11 so as to be attachable thereto and detachable therefrom . a fitting recessed part 12 is formed at the lower end of the main body 10 . the fitting protrusion 9a is fitted into the fitting recessed part 12 , and thus the main body 10 can be connected to the output shaft 8 of the motor 7 so as to be attachable thereto and detachable therefrom . a cylindrical part 13 is formed on the lower end of the main body 10 . a groove 14 of predetermined length is bored along the peripheral face of a peripheral wall 13b on the horizontal plate part 13a of the cylindrical part 13 . a shutter 15 consists of a shield part 15a having a peripheral face with the same curvature as that of the peripheral face of the peripheral wall 13b , and an engaging part 15b extending in a direction orthogonal therewith . the inner peripheral face of the shield part 15a slidingly abuts against the outer peripheral face of the peripheral wall 13b . the lower face of the engaging part 15b slidingly abuts against the horizontal plate part 13a of the cylindrical part 13 . a screw 17 is inserted into a screw inserting hole which opens on a plate like positioning member 16 . said screw 17 is threaded into a threaded hole 18 bored on the engaging part 15b . when the screw 17 is unfastened from the threaded hole 18 , the shutter 15 can slide along the outer peripheral face of the cylindrical part 13 . the screw 17 is tightly fastened or threaded into the threaded hole 18 so that the shutter 15 can be fixed in an appropriate position on the cylindrical part 13 . a light transmitting type object detecting sensor 19 is disposed on the base plate 6 . the peripheral wall 13b of the penstocker main body 10 is arranged in its detecting recessed part . on the peripheral wall 13b are provided slits 20 , 21 and 22 which are located in the movement path of the shield part 15a of the shutter 15 and serve to encode the presence or absence of the penstocker , the kind of pens held , an origin of the penstocker , etc . a predetermined number of slits 21 , 21 and 22 are provided on the peripheral wall 13b . penstocker main body 10 is placed at a predetermined pen change position by rotating the penstocker main body 10 using the motor . thus , the pen holding portion located at this pen change position and a drawing head ( not shown ) of an automatic drafting machine can mutually exchange pens . in the figures , 23 denotes a positioning mark and 24 a positioning line . in the penstocker of the above described construction , the shield part 15a of the shutter 15 may be set to a position shown in fig3 ( a ). the penstocker main body 10 is rotated , using the motor 7 , so as to move the slits 20 , 21 and 22 on peripheral wall 13b to a position adjacent to the detecting recessed part 19a of the sensor 19 . then , the sensor 19 detects the presence or absence of slits 20 , 21 and 22 . a controller recognizes the origin of the penstocker main body 10 using slit 20 , and that the kind of a pen held by the penstocker main body 10 belongs to one of three kinds by using the slits 21 and 22 . when kinds 1 to 3 are respectively input in the controller of the automatic drafting machine as that of a pencil , a ball pen and a ceramic pen , the controller at the position of fig3 ( a ) recognizes the type of pen and can further discriminate whether the penstocker main body 10 is a penstocker to be designated for use in plotting or drafting for example . when the sensor 19 detects the slits 20 and 21 in fig3 ( b ), by shielding the slit 22 by the shutter 15 , the controller recognizes the origin and the stocker of a first particular kind . when the sensor 19 detects the slits 20 , 21 and 22 in fig3 ( a ), whereby none of the slits are shielded , the controller recognizes the origin and the stocker of a second particular kind . when the sensor 19 detects only the slit 20 in fig3 ( c ), by shielding or covering the slits 21 and 22 by the shutter 15 , the controller recognizes the origin and the stocker of a third particular kind . thus , the slit 20 located at the right end on the peripheral wall 13b in fig3 a - 3c indicates the origin , and the two slits 21 , 22 adjacent to the slit 20 indicates the kind of pen being used . when only one slit 20 is exposed , the origin is determined and the pen is determined to be of a third particular kind . when all three slits 20 , 21 and 22 are exposed , the origin is determined and the pen is determined to be of a second particular kind . when two slits 20 and 21 are exposed , the origin is determined and the pen is determined to be of a first particular kind . when the sensor 19 fails to detect the presence of slits , it is determined that there is no penstocker . the shutter 15 may be fixed by fastening the screw 17 as shown in fig1 or by engaging a protrusion 26 of a positioning member 25 made of an elastic material into a locking recessed part 27 provided at a predetermined position on the lower face of the horizontal plate part 13a as shown in fig4 and 5 . in the penstocker shown in fig4 and 5 , the movement of the shutter 15 is conducted such that an external movement force is manually applied on the shutter 15 , the protrusion 26 of the positioning member 25 being detached from the locking recessed part 27 , the screw 17 being moved along the groove 14 together with the shutter 15 . since the penstocker of the present invention has the above described construction , many different kinds of pens can be readily discriminated by a single stocker . furthermore , since a user can easily change a slit pattern , it is not necessary to prepare many penstockers when many kinds of pens having the same configuration are employed .