the preferred embodiments of the present invention are further described in detail with the following embodiment and the accompanying drawings . fig1 to 10 illustrate the construction of an embodiment of the present invention . as shown in fig1 and 2 , the embodiment comprises a casing which is composed of an outer casing 1 , a fixed blade holder 2 and a retractable blade holder 3 , blades 4 , a reset spring 5 , a pushbutton 6 , a spring 7 , a plug 8 and a blade changer 9 . blades 4 are short blades each with an aperture 41 and are for industrial hand - held cutters , and in this embodiment , blades of number 11 are used , while in other embodiments , other sizes of blades may be used . in this embodiment , the head of the fixed blade holder 2 is disposed with a plane surface 21 , and a projection 22 is disposed on the plane surface 21 , and by inserting the projection 22 into the aperture 41 of one of the blades 4 , the blade 4 can be fixed securely on the head of the fixed blade holder 2 . the fixed blade holder 2 is disposed with a hollow 23 that fixes the blade 4 on the head of the fixed blade holder 2 , and the shape and dimension of the hollow 23 corresponds to the shape and dimension of the rear half of the blade 4 . the retractable blade holder 3 is disposed with a plane surface 31 which faces the plane surface 21 of the fixed blade holder 2 , the plane surface 31 of the retractable blade holder 3 can slide on the plane surface 21 of the fixed blade holder 2 . the bottom of the retractable blade holder 3 is disposed with a protrusion 32 that engages with one end of the reset spring 5 , and the bottom or the part near the bottom of the fixed blade holder 2 is disposed with a protrusion 25 that engages with the other end of the reset spring 5 . the fixed blade holder 2 is disposed with a receiving slot 26 that provides a cavity for the movement of the reset spring 5 . the retractable blade holder 3 can retract and restore to its original position as against the fixed blade holder 2 by means of the reset spring 5 . in this embodiment , the fixed blade holder 2 and the retractable blade holder 3 are each semielliptical in shape respectively , and when the plane surface 31 of the retractable blade holder 3 engages with the plane surface 21 of the fixed blade holder 2 , they together are in the shape of an ellipse , and in other embodiments , the fixed blade holder 2 and the retractable blade holder 3 can be semicircular in shape respectively , and when the plane surface 31 of the retractable blade holder 3 engages with the plane surface 21 of the fixed blade holder 2 , they together are in the shape of a circle , or alternatively the fixed blade holder 2 and the retractable blade holder 3 can be in any other shape . these are not shown in the accompanying drawings . an opening 27 is disposed on one side of the bottom or the part near the bottom of the fixed blade holder 2 , the pushbutton 6 is disposed with an inserting member for connection 61 which has a fastening block 62 on the surface near the pushbutton 6 , a spring 7 is disposed in the center of the inserting member for connection 61 , one end of the spring 7 is exposed from the inserting member for connection 61 , and the inserting member for connection 61 together with the spring 7 is inserted into and connected with the opening 27 . the inserting member for connection 61 is disposed with a positioning block 63 , the opening 27 is disposed with a corresponding limiting groove 24 , and the positioning block 63 is inserted into the limiting groove 24 to limit the pushbutton 6 not to rotate sidewise . the bottom of the outer casing 1 is disposed with a sliding groove 11 which has at least two positioning slots 12 , 13 , and in this embodiment , the sliding groove 11 is disposed with three positioning slots 12 , 13 , 14 , the fixed blade holder 2 and the retractable blade holder 3 are entirely and completely inserted from the bottom of the outer casing 1 into the outer casing 1 , the inserting member for connection 61 is partly inserted into the sliding groove 11 , the pushbutton 6 is exposed from the outer casing 1 outside the sliding groove 11 . an inserting groove 81 is disposed along the outer edge of the bottom of the plug 8 , the bottom of the outer casing 1 is disposed with a corresponding inserting part 15 , and the plug 8 removably engages with the bottom of the outer casing 1 to prevent the fixed blade holder 2 , the retractable blade holder 3 , the blade 4 and the pushbutton 6 from sliding out from the bottom of the outer casing 1 . the sliding of the inserting member for connection 61 in the sliding groove 11 actuates the fixed blade holder 2 and the retractable blade holder 3 to slide in the outer casing 1 , by means of the movement of the spring 7 the fastening block 62 of the inserting member for connection 61 can removably engage with the positioning slots 12 , 13 , 14 , and the fixed blade holder 2 and the retractable blade holder 3 can slide in the outer casing 1 till their heads are exposed from the head of the outer casing 1 . the pushbutton 6 is in the shape of a clip which can clip onto another object . in this embodiment , the outer casing 1 is in the shape of an elliptical cylinder , while in other embodiments which are not shown in the accompanying drawings , the outer casing 1 can be in the shape of a circular cylinder , rectangular cylinder or in any other shape . in this embodiment , the sliding groove 11 is disposed with three positioning slots 12 , 13 , 14 , and the first positioning slot 12 is disposed near the bottom of the outer casing 1 , the second positioning slot 13 is disposed in the middle section of the sliding groove 11 and the third positioning slot 14 is disposed near the other end of the sliding groove 11 . as shown in fig7 , when the fastening block 62 of the inserting member for connection 61 engages with the first positioning slot 12 , the fixed blade holder 2 and the retractable blade holder 3 are entirely and completely disposed inside the outer casing 1 and the blade 4 is not exposed from the outer casing 1 . as shown in fig8 , when the fastening block 62 of the inserting member for connection 61 engages with the second positioning slot 13 , the blade 4 is exposed from the head of the outer casing 1 and only a small section of each of the heads of the fixed blade holder 2 and the retractable blade holder 3 is exposed from the head of the outer casing 1 . as shown in fig9 , when the fastening block 62 of the inserting member for connection 61 engages with the third positioning slot 14 , the heads of the fixed blade holder 2 and the retractable blade holder 3 together with the blade 4 are exposed from the head of the outer casing 1 . the length of the exposed sections is 20 centimeters , and in other embodiments , the length of the exposed sections can be of any other length . in other embodiments , which are not shown in the accompanying drawings , the sliding groove 11 is disposed with two positioning slots , and the first positioning slot is disposed near the bottom of the outer casing 1 while the second positioning slot is disposed near the other end of the sliding groove 11 . when the fastening block 62 of the inserting member for connection 61 engages with the first positioning slot , the fixed blade holder 2 and the retractable blade holder 3 are entirely and completely disposed inside the outer casing 1 and the blade 4 is not exposed from the outer casing 1 . when the fastening block 62 of the inserting member for connection 61 engages with the second positioning slot , the heads of the fixed blade holder 2 and the retractable blade holder 3 together with the blade 4 are exposed from the head of the outer casing 1 . as illustrated in fig1 to 5 , in this embodiment , the blade changer 9 is in the shape of a box , and in other embodiments which are not shown in the accompanying drawings , the blade changer 9 can be in any other shape . the blade changer 9 is divided into a blade unloading chamber 91 and a blade loading chamber 92 , each chamber is disposed with an inserting cavity 911 , 921 near the top , the shape and dimension of each inserting cavity 911 , 921 corresponds to the shape and dimension of the inserting sections of the fixed blade holder 2 and the retractable blade holder 3 . the lower half of each inserting cavity 911 , 921 is disposed with a stopping block 912 , 922 . the fixed blade holder 2 must be positioned on the top and the retractable blade holder 3 must be positioned at the bottom so that they together can be inserted into the inserting cavity 911 , 921 , and the stopping block 912 , 922 can just block the front end of the retractable blade holder 3 , thereby causing the retraction of the retractable blade holder 3 . the blade loading chamber 92 is disposed with a limiting member 923 which only allows horizontal overlapping placement of a plurality of new blades 4 , the end of the new blades 4 having the aperture 41 is positioned near the stopping block 922 , and the blade changer 9 has a bottom cover 93 which is disposed with a compression spring 94 in position that corresponds to the new blades 4 , the compression spring 94 exerts force upward against the new blades 4 so that the topmost new blade 4 is just higher than the stopping block 922 . as shown in fig4 and 5 , the blade unloading chamber 91 is disposed with a stopping plate 95 at the inserting cavity 911 , the stopping plate 95 is connected with and disposed inside the blade unloading chamber 91 by a torsion spring 96 , the shorter end 961 of the torsion spring 96 is disposed in the depression 952 of the stopping plate 95 , the longer end 962 of the torsion spring 96 leans against the plane surface 915 , and the rotational shaft end 951 of the stopping plate 95 that is near the torsion spring 96 is disposed in the depression 913 of the blade unloading chamber 91 . the stopping plate 95 removably covers the inserting cavity 911 , and when the fixed blade holder 2 is inserted into the inserting cavity 911 , the stopping plate 95 is pushed open , and when the fixed blade holder 2 is removed from the inserting cavity 911 , the force of the torsion spring 96 actuates the stopping plate 95 to restore to its original position immediately , the stopping plate 95 therefore prevents the old blades from failing out from the inserting cavity 911 . the upper half of the inserting cavity 921 of the blade loading chamber 92 is disposed with a depression 924 of size and dimension corresponding to the shape and dimension of the inserting section of the fixed blade holder 2 so as to fix the position of the inserting section of the fixed blade holder 2 , thereby allowing the projection 22 to precisely pass through the aperture 41 of the topmost new blade 4 to fix the new blade 4 on the head of the fixed blade holder 2 . the upper half of the inserting cavity 911 of the blade unloading chamber 91 is disposed with a depression 914 of size and dimension corresponding to the shape and dimension of the inserting section of the fixed blade holder 2 so as to fix the position of the inserting section of the fixed blade holder 2 , thereby allowing easy unloading of the old blade 4 . the bottom cover 93 can removably or securely engage with the bottom of the blade changer 9 . the bottom cover 93 can be disposed in position that corresponds to the blade unloading chamber 91 with one or a plurality of magnets or a magnetic layer to attract the unloaded blades 4 at the bottom , which is not shown in the accompanying drawings . as shown in fig7 , when the embodiment is in a standby state , the fastening block 62 of the inserting member for connection 61 engages with the first positioning slots 12 , the fixed blade holder 2 and the retractable blade holder 3 are entirely and completely disposed inside the outer casing 1 and the blade 4 is not exposed from the outer casing 1 . the user can carry around or store the outer casing 1 for use , for example , by inserting the pushbutton 6 into the inner pocket of a tool box or a shoulder bag or the like , and the blade changer 9 can also be stored for use . as shown in fig8 , to use the present invention , the fastening block 62 of the inserting member for connection 61 engages with the second positioning slot 13 , the blade 4 is exposed from the head of the outer casing 1 , and a large section or only a small section of each of the heads of the fixed blade holder 2 and the retractable blade holder 3 is exposed from the head of the outer casing 1 . the user can use the sharp pointed end of the blade 4 to cut objects . as shown in fig7 , after finishing using , the user only needs to engage the fastening block 62 of the inserting member for connection 61 with the first positioning slot 12 . as shown in fig9 , when the pointed end of the blade 4 has become blunt and it is required to be replaced with a new blade 4 , the user only needs to engage the fastening block 62 of the inserting member for connection 61 with the third positioning slot 14 , the heads of the fixed blade holder 2 and the retractable blade holder 3 together with the blade 4 are exposed from the head of the outer casing 1 . then , insert the heads of the fixed blade holder 2 and the retractable blade holder 3 together with the blade 4 into the inserting cavity 911 of the blade unloading chamber 91 , the fixed blade holder 2 must be positioned on the top and the retractable blade holder 3 must be positioned at the bottom so that they together can be inserted into the inserting cavity 911 , otherwise the stopping block 912 will block the front end of the fixed blade holder 2 and they cannot be inserted . when the heads of the fixed blade holder 2 and the retractable blade holder 3 together with the blade 4 are inserted , the stopping plate 95 is pushed open , the stopping block 912 can just block the front end of the retractable blade holder 3 , thereby causing the retraction of the retractable blade holder 3 as against the fixed blade holder 2 , and the blade 4 is therefore entirely exposed and simultaneously it falls onto the bottom cover 93 owing to gravity , and if the corresponding position on the bottom cover 93 is disposed with one or a plurality of magnets or a magnetic layer , the unloaded blade 4 will be attracted at the bottom . the user then pulls out the fixed blade holder 2 and the retractable blade holder 3 , the force of the torsion spring 96 actuates the stopping plate 95 to restore to its original position immediately , the stopping plate 95 therefore blocks the inserting cavity 911 and prevents the old blades 4 from falling out from the inserting cavity 911 . the user then inserts the heads of the fixed blade holder 2 and the retractable blade holder 3 into the inserting cavity 921 of the blade loading chamber 92 , the stopping block 922 can just block the front end of the retractable blade holder 3 , thereby causing the retraction of the retractable blade holder 3 as against the fixed blade holder 2 , the plane surface 21 of the fixed blade holder 2 is therefore exposed . as the compression spring 94 of the bottom cover 93 exerts force upward , the projection 22 on the plane surface 21 of the fixed blade holder then passes through the aperture 41 of the topmost new blade 4 , and as the topmost new blade 4 is just higher than the stopping block 922 , when the user pulls out the fixed blade holder 2 and the retractable blade holder 3 , the plane surface 31 of the retractable blade holder 3 will immediately fix the topmost new blade 4 in between the plane surfaces 21 , 31 of the fixed blade holder 3 and the retractable blade holder 3 . therefore , during the whole process of changing the blades , the user is not required to touch the blades directly and the whole process is completely automatic . when all the overlapping horizontally placed new blades 4 in the blade loading chamber 92 are used up , the user can dispose of the whole blade changer 9 , and there is no need to take out from the blade changer 9 the old blades 4 that requires to be disposed of . for the purposes of protecting the natural environment , the user can also open the bottom cover 93 to dispose of the old blades 4 and place some new blades 4 , and the blade changer 9 can be reused . the above embodiment is a preferred embodiment of the present invention . the present invention is capable of other embodiments and is not limited by the above embodiment . any other variation , decoration , substitution , combination or simplification , whether in substance or in principle , not deviated from the spirit of the present invention , is replacement or substitution of equivalent effect and falls within the scope of protection of the present invention .