hereinafter , embodiments of this invention will be described with reference to the drawings . fig1 is a sectional view of a printing mechanism according to the invention and fig2 is a perspective view of the inventive printing apparatus . in these figures , a platen 2 has a plurality of opening holes 11a having relatively small diameters , a plurality of large openings 11b for driving rollers 3 which feed a recording medium 6 , and a flat section 11c extending on a front face of platen 2 over a region corresponding to a printing span of a head 1 . a vacuum space 11d is provided below platen 2 . the vacuum space 11d communicates with the flat section 11c on the front face of the platen 2 through the plural opening holes 11a effective to attract recording medium 6 . the width of vacuum space 11d and the width of the region occupied by opening holes 11a are set identical to the maximum width size of a recording medium which can be printed by the printing apparatus . in this embodiment , the width is set at about 841 mm for a0 size paper . as shown in fig3 or 4 , the opening holes 11a are arranged such that total opening area a of the opening holes 11a per unit surface area w is gradually reduced with increasing distance from a guide 11e which determines the widthwise position of recording medium 6 . in the fig3 embodiment , the opening holes 11a have a diameter of 3 mm and are arranged at a pitch l 1 = 20 mm within a span corresponding to an a3 size recording medium . then , the opening holes 11a are arranged at a pitch l 2 = 30 mm beyond the a3 size span and within the size a2 span . further , the opening holes 11a are arranged at a pitch l 3 = 40 mm beyond the a2 size span and within a0 size span . in the fig4 embodiment , all of the opening holes 11a are arranged at a pitch of l 1 = l 2 = l 3 = 20 mm , but the diameter of opening holes 11a is gradually changed in correspondence to the widthwise span of different recording medium sizes . in detail , the diameter of holes 11a is set at 3 mm within a span of a3 size recording medium , then is set at 2 . 35 mm beyond the a3 span and within the a2 span , and further is set at 1 . 86 mm beyond the a2 span and within the a0 span . on the other hand , the opening holes 11a are arranged in the recording medium feeding direction ( sub - scanning direction ) such as to sufficiently cover a range opposed to multi - nozzle arrangement face 7b of the head 1 . according to one embodiment , the total length of the multi - nozzle arrangement is set to 8 mm since 64 nozzles are arranged vertically at a pitch of 1 / 8 mm . namely , the opening holes are arranged in the sub - scanning direction at a common pitch of 20 mm and extend in that direction over two pitch intervals so as to cover a range from minimum 20 mm to maximum 40 mm , as shown in fig3 and 4 . the vacuum means may include a fan 5 which sucks air from the vacuum space 11d inside the platen 2 . reverting to fig1 and 2 , each driving roller 3 sandwiches the recording medium 6 with a pinch roller 8 to feed medium 6 in the sub - scanning direction . a carriage 13 carries the head 1 , and is supported by guide shafts 14 and is driven in a direction ( main scanning direction ) transverse to the feeding direction by means of a head - feeding servo motor , etc . ( not shown ) through a wire or belt and a spool ( not shown ) so as to undergo reciprocating movement . in the present embodiment , the recording medium 6 is fed from a rear part to a front part of the printing apparatus by means of the driving rollers 3 and pinch rollers 8 . the driving rollers 3 receive a drive force from a pulse motor ( not shown ) through a timing belt and a spool . an ink supply tube 12 supplies ink to the head 1 from an ink cartridge ( not shown ). a flexible circuit substrate 15 applies to each nozzle of the head 1 of a driving pulse based on printing data . as described before , the head 1 is a multi - nozzle ink jet recording head of the on - demand type . sixty - four nozzles are linearly arranged in the sub - scanning direction at a pitch of 8 nozzles / mm and operate to eject ink droplets toward recording medium 6 on a demand basis in response to drive pulses fed from the flexible circuit substrate 15 according to printing data . the next description is given for scanning operation of the above constructed embodiment of the inventive printing apparatus . the recording medium 6 is set in an initial position such that a top edge thereof is sandwiched between the driving rollers 3 and the pinch rollers 8 in the sub - scanning direction , and a rear part thereof passes between head 1 and flat section 11c of platen 2 , which defines a guide face of the recording medium 6 , and rearwardly of the apparatus in a free or unconfined , state . then , the fan 5 is operated to initiate suction in space 11d . by this suction , air is evacuated from the vacuum space 11d beneath platen 2 and is expelled from space 11d through fan 5 . consequently , the pressure in space 11d is reduced such that air flows from above platen 2 along the recording medium 6 into the space 11d through the opening holes 11a so as to generate attractive forces . due to these attractive forces generated in the opening holes 11a , the recording medium 6 starts to closely contact the flat section 11c of platen 2 . by this contact , many of the opening holes 11a are closed to reduce the quantity of air flowing through opening holes 11a to space 11d beneath platen 2 . consequently , air pressure is abruptly reduced in a gap between the recording medium 6 and the platen 2 to thereby boost the attractive forces . with increase of the attractive forces , the recording medium 6 is made to closely contact the flat section 11c of platen 2 such that the flatness of medium 6 becomes identical to that of the flat section 11c to prevent floating of recording medium 6 . the flat section 11c of the platen 2 is precisely finished to achieve flatness at an accuracy of less than 0 . 2 mm over an entire area within the printing span of head 1 , and moreover platen 2 has a sufficient stiffness to avoid deformation such as bending due to the suction . generally in ink jet printing , it is necessary to maintain the distance between the head 1 and the platen 2 in the order of 1 . 0 mm - 1 . 2 mm . according to the present invention , recording medium 6 can be stably placed with a variation comparable to the variation of the flatness of platen 2 . the next description is given for operation after completion of the attraction of recording medium 6 . while displacing carriage 13 in the main scanning direction in response to a printing start signal , the recording head 1 mounted on carriage 13 is operated to eject ink droplets to effect printing according to print data . the printed pattern and ink jet amount can be determined according to the print data fed through the flexible circuit substrate or cable . the printing interval in the sub - scanning direction is determined by the total number of nozzles and the pitch thereof arranged on the head 1 . in this embodiment , the printing interval is set to 8 mm as described before . accordingly with each line scanning of the carriage 13 in the main scanning direction , the recording medium 6 is intermittently fed forwardly of the printing apparatus by an 8 mm step through the driving rollers 3 and the pinch rollers 8 . at this time , attractive forces at the opening holes 11a applied to the recording medium 6 can produce appropriate tension , or resistance to the feeding of the recording medium so as to facilitate stable feeding . during the course of the printing operation in the manner as described above , the recording medium 6 can be continuously attracted through the edge thereof to avoid any drawback such as floating . the next description is given for printing recording media of different widths in the inventive printing apparatus . the attractive forces on the recording medium can be boosted by closing the opening holes 11a with the recording medium 6 so as to efficiently avoid floating . therefore , when inserting a recording medium 6 having a relatively small width , such as a4 size and a3 size , into the printing apparatus which has a relatively large maximum printing span covering , for example , a0 size of 841 mm in this embodiment , a 1 / 2 to 3 / 4 of the opening holes 11a are not covered by the recording medium , thereby failing to generate strong attractive forces . in the present invention , in order to compensate for the resulting potential drawback , the opening holes are arranged such that the total opening area thereof per unit surface area of platen 2 is gradually decreased in the widthwise direction of the recording medium , from edge guide 11e , so as to avoid a considerable reduction of the attractive forces even when some of the opening holes are not covered by narrow recording medium 6 . in detail , the pitch of opening holes 11a is increased with increasing distance from edge guide 11e , or the diameter of opening holes 11a is reduced accordingly to effect the compensation . fig3 and 4 show examples of such arrangements and structures . in these embodiments , the total opening area per unit surface area w = 2500 mm 2 , is gradually changed such that the total opening area in a unit surface area is set to about 92 mm 2 within a span of a3 size , then to about 56 . 5 mm 2 within a span of a2 size , and further to about 35 mm 2 within a span of a0 size . according to experimental results obtained with the inventive printing apparatus having opening holes arranged as described above , the attractive force applied to a narrow recording medium 6 is reduced only by 20 % as compared to a wider recording medium which can cover all of the opening holes 11a thereby efficiently ensuring the application of effective attraction forces to the narrower recording medium 6 against the face of platen 2 to avoid floating . consequently , good printing can be carried out for recording media of various sizes without degradation of print quality . as described above , according to the present invention , a recording medium can be closely contacted onto a flat section of platen within a region corresponding to the printing span of a head through attractive forces produced by vacuum means and opening holes having relatively small diameters to avoid floating , thereby achieving the effect that the distance between the nozzle tips of the head and the recording medium is controlled and maintained constant across the entire multi - nozzle arrangement . consequently , an ink jet recording apparatus can be provided such that reduction in targeting accuracy of ink droplets can be prevented to obtain high quality of print image with highly accurate dot positioning . while the description above refers to particular embodiments of the present invention , it will be understood that many modifications may be made without departing from the spirit thereof . the accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention . the presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive , the scope of the invention being indicated by the appended claims , rather than the foregoing description , and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein .