Patent Publication Number: US-11396203-B2

Title: Knock type writing tool

Description:
TECHNICAL FIELD 
     The present invention pertains to a knock type writing tool including a writing body in which a thermochromic ink is contained. 
     BACKGROUND ART 
     Conventionally, there has been known a knock type writing tool in which a pen tip projects or retracts every time a knock member is pressed forward. 
     Such a knock type ballpoint pen has been disclosed in Patent Document 1. The knock type ballpoint pen is provided with a rotating cam member configured to cause a pen tip to project and retract. A protrusion part, which is configured to be inserted into a rear end opening of an ink containing tube, and an abutment part, which is configured to abut on a rear end of the ink containing tube, are provided on a front surface of the rotating cam member. A tapered press part, which is configured to be pressed onto an inner circumferential edge portion of the rear end opening of the ink containing tube, is provided on an outer surface of the protrusion part. Thus, the outer surface of the protrusion part ahead of the press part is in a noncontact state with an inner surface of the ink containing tube. In addition, a cutout part extending in a front and rear direction is provided at a portion of the press part, and a concave part communicating with the cutout part is provided at a portion of the abutment part. This, under a state wherein the abutment part of the rotating cam member and the rear end of the ink containing tube are in contact with each other, due to the cutout part and the concave part, an air passage is formed such that the inside of the ink containing tube and the outside thereof are communicated. Thus, air replacement for an ink in the ink containing tube is made smoother, and thus smoother ink discharging performance can be obtained. 
     Furthermore, recently, there has been known a knock type writing tool in which a pen tip is rotated every time a knock member is pressed forward, in addition to the feature wherein the pen tip projects and retracts. Such a writing tool, whose pen tip is rotated, is popular in the market because wear of the pen tip can be equalized. 
     Such a knock type writing tool has been disclosed in Patent Document 2. In the writing tool, a refill is rotated in conjunction with a rotation of a rotating cam for the purpose of avoiding uneven wear of a pen tip. In addition, in the structural example shown in Patent Document 2, an engaged part carved in a V shape in a circumferential direction is formed at a head end surface of a tail plug of the refill, and an engaging part carved in an inversed V shape in the circumferential direction is formed at a leading end surface of the rotating cam so as to engage with the engaged part. 
     PRIOR ART DOCUMENT 
     Patent Document List 
     Patent Document 1 is JP-A-2004-17397. 
     Patent Document 2 is JP-A-H11(1999)-78361. 
     SUMMARY OF INVENTION 
     Technical Problem 
     When a thermochromic ink is used, an amount of the ink discharged from a pen tip is relatively larger. Thus, when the structure disclosed in Patent Document 1 is applied to a writing tool using a thermochromic ink, air may not be taken sufficiently from the outside of the ink containing tube into the inside thereof, so that smooth ink discharging performance from the pen tip may not be obtained (interruption of handwriting may be caused). 
     In the same way, when the structure disclosed in Patent Document 2 is applied to a writing tool using a thermochromic ink, it is difficult to provide a large air passage which communicates the inside of the refill with the outside thereof, so that smooth ink discharging performance from the pen tip may not be obtained (interruption of handwriting may be caused). 
     An object of the present invention is to solve the above problems. That is to say, an object of the present invention is to provide a knock type writing tool using a thermochromic ink in which smooth ink discharging performance from a pen tip is obtained and in which the pen tip is rotated every time a knock member is operated. 
     In the present disclosure, the term “front” means a pen-tip side, and the term “rear” means an opposite side thereof. 
     Solution to Problem 
     According to the first aspect of the present invention, a knock type writing tool is provided, the knock type writing tool including: a writing body having a pen tip at a front end thereof, a thermochromic ink in an inside thereof, and an air hole at a rear end thereof, the air hole communicating with a space in which the thermochromic ink is contained (a follower may be interposed in the middle); a shaft cylinder configured to contain the writing body in such a manner that the writing body is movable in a front and rear direction with respect to the shaft cylinder; a knock member provided on an outer surface of the shaft cylinder in such a manner that the knock member is movable in a front and rear direction with respect to the shaft cylinder; a projection and retraction rotating member configured to move in a front and rear direction to cause the pen tip to project from or retract within a front end of the shaft cylinder and configured to cause the writing body to rotate, every time the knock member is moved forward; and a cam part provided in the shalt cylinder, the cam part being able to engage or disengage with the projection and retraction rotating member depending on a position of the knock member; wherein a plurality of projection pieces, each of which is projected rearward, are provided at a rear end of the writing body, the plurality of projection pieces being arranged circumferentially apart from each other; a plurality of gaps between the plurality of projection pieces communicate with the air hole; a front portion of the projection and retraction rotating member is provided with a plurality of abutment parts, on which respective rear ends of the plurality of projection pieces are to abut, and with a plurality of protrusion parts, each of which is protruded more forward than the plurality of abutment parts; the plurality of protrusion parts are respectively inserted in the plurality of gaps between the plurality of projection pieces; and a portion of each of the plurality of gaps is left in front of each of the plurality of protrusion parts. 
     According to the above structure, the plurality of gaps between the plurality of projection pieces can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. 
     For example, the shalt cylinder is configured to contain the writing body in such a manner that the writing body is movable in the front and rear direction with respect to the shaft cylinder under a state in which the writing body is biased rearward; the projection and retraction rotating member is configured to move in the front and rear direction, in conjunction with a movement of the knock member in the front and rear direction; and the projection and retraction rotating member is configured to move forward together with the knock member and to cause the pen tip to project from or retract within the front end of the shaft cylinder, every time the knock member is moved forward. 
     In this case, as the knock member is also affected by a force for biasing the writing body rearward, the knock member can be automatically returned, which can provide a good operation feeling and a good operability. 
     In addition, it is preferable that the plurality of projection pieces and the plurality of protrusion parts are configured to circumferentially engage with each other in such a manner that the writing body rotates in conjunction with a rotation of the projection and retraction rotating member. 
     According to this feature, although the structure is simple, circumferential engagement of the projection and retraction rotating member with the writing body can be surely obtained. 
     In addition, in this case, it is further preferable that engagement surfaces of the plurality of projection pieces against the plurality of protrusion parts and engagement surfaces of the plurality of protrusion parts against the plurality of projection pieces extend substantially in parallel in a front and rear direction. 
     According to this feature, the respective engagement surfaces are engaged more efficiently, so that the circumferential engagement of the projection and retraction rotating member with the writing body can be obtained more surely. 
     In addition, it is preferable that a front portion of the projection and retraction rotating member is connected to an annular auxiliary protrusion part provided integrally with the plurality of protrusion parts. 
     According to this feature, the plurality of protrusion parts are effectively reinforced, which can prevent breakage of the plurality of protrusion parts or the like. 
     In addition, it is preferable that an inside surface of the annular auxiliary protrusion part is a tapered auxiliary abutment surface, and a rear end portion of the writing body is provided with an auxiliary tapered portion on which the auxiliary abutment surface is to abut. 
     According to this feature, the rear end portion of the writing body can be supported more stably by using the tapered auxiliary abutment surface. Furthermore, since the auxiliary abutment surface and the auxiliary tapered portion are tapered, a so-called “centering” effect can be obtained. 
     In addition, it is preferable that the plurality of projection pieces are formed of polypropylene resin or polyethylene resin. 
     According to this feature, the plurality of projection pieces can have sufficient strength, so that bending or buckling thereof can be effectively prevented. Thus, a stable rotation of the writing body can be obtained in conjunction with a rotation of the projection and retraction rotating member. 
     In addition, according to the second aspect of the present invention, a knock type writing tool is provided, the knock type writing tool including: a writing body having a pen tip at a front end thereof, a thermochromic ink in an inside thereof, and an air hole at a rear end thereof, the air hole communicating with a space in which the thermochromic ink is contained (a follower may be interposed in the middle); a shaft cylinder configured to contain the writing body in such a manner that the writing body is movable in a front and rear direction with respect to the shaft cylinder; a knock member provided on an outer surface of the shaft cylinder in such a manner that the knock member is movable in a front and rear direction with respect to the shaft cylinder; a projection and retraction rotating member configured to move in a front and rear direction to cause the pen tip to project from or retract within a front end of the shaft cylinder and configured to cause the writing body to rotate, every time the knock member is moved forward; and a cam part provided in the shaft cylinder, the cam part being able to engage or disengage with the projection and retraction rotating member depending on a position of the knock member; wherein a plurality of protrusion parts, each of which is protruded forward, are provided at a front portion of the projection and retraction rotating member, the plurality of protrusion parts being arranged circumferentially apart from each other; a rear end of the writing body is provided with a plurality of abutment parts, on which respective front ends of the plurality of protrusion parts are to abut, and with a plurality of projection pieces, each of which is projected more rearward than the plurality of abutment parts; the plurality of projection pieces are respectively inserted in a plurality of gaps between the plurality of protrusion parts; and a portion of each of the plurality of gaps is left behind each of the plurality of projection pieces. 
     According to the above structure, the plurality of gaps between the plurality of protrusion parts can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. 
     In addition, according to the third aspect of the present invention, a knock type writing tool is provided, the knock type writing tool including: a writing body having a pen tip at a front end thereof, a thermochromic ink in an inside thereof, and an air hole at a rear end thereof, the air hole communicating with a space in which the thermochromic ink is contained (a follower may be interposed in the middle); a shaft cylinder configured to contain the writing body in such a manner that the writing body is movable in a front and rear direction with respect to the shaft cylinder; a knock member provided on an outer surface of the shaft cylinder in such a manner that the knock member is movable in a front and rear direction with respect to the shaft cylinder; a projection and retraction rotating member configured to move in a front and rear direction to cause the pen tip to project from or retract within a front end of the shaft cylinder and configured to cause the writing body to rotate, every time the knock member is moved forward; and a cam part provided in the shaft cylinder, the cam part being able to engage or disengage with the projection and retraction rotating member depending on a position of the knock member; wherein a plurality of protrusion parts, each of which is protruded forward, are provided at a front portion of the projection and retraction rotating member; a rear end of the writing body is provided with a flexible member which is elastically deformable; the flexible member is made of a porous material; and the plurality of protrusion parts and the flexible member are configured to circumferentially engage with each other when the plurality of protrusion parts cause the flexible member to elastically deform and sink in an inside of the flexible member in such a manner that the writing body rotates in conjunction with a rotation of the projection and retraction rotating member. 
     According to the above structure, due to the flexible member made of a porous material, air permeability can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. In addition, the flexible member can absorb an impact and/or a force applied to the writing body at the time of writing, and thus it is possible to provide a user with a soft and comfortable writing feeling. 
     In addition, according to the fourth aspect of the present invention, a knock type writing tool is provided, the knock type writing tool including: a writing body having a pen tip at a front end thereof, a thermochromic ink in an inside thereof, and an air hole at a rear end thereof, the air hole communicating with a space in which the thermochromic ink is contained (a follower may be interposed in the middle); a shaft cylinder configured to contain the writing body in such a manner that the writing body is movable in a front and rear direction with respect to the shaft cylinder; a knock member provided on an outer surface of the shaft cylinder in such a manner that the knock member is movable in a front and rear direction with respect to the shaft cylinder; a projection and retraction rotating member configured to move in a front and rear direction to cause the pen tip to project from or retract within a front end of the shaft cylinder and configured to cause the writing body to rotate, every time the knock member is moved forward; and a cam part provided in the shaft cylinder, the cam part being able to engage or disengage with the projection and retraction rotating member depending on a position of the knock member; wherein a plurality of protrusion parts, each of which is protruded forward, are provided at a front portion of the projection and retraction rotating member; a rear end of the writing body is provided with a flexible member which is elastically deformable; the flexible member is provided with a connection air hole configured to communicate with the air hole, the connection air hole being opened at a rear end or a lateral surface of the flexible member; and the plurality of protrusion parts and the flexible member are configured to circumferentially engage with each other when the plurality of protrusion parts cause the flexible member to elastically deform and sink in an inside of the flexible member in such a manner that the writing body rotates in conjunction with a rotation of the projection and retraction rotating member. 
     According to the above structure, the connection air hole can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. 
     In addition, according to the fifth aspect of the present invention, a knock type writing tool is provided, the knock type writing tool including: a writing body having a pen tip at a front end thereof, a thermochromic ink in an inside thereof, and an air hole at a rear end thereof, the air hole communicating with a space in which the thermochromic ink is contained (a follower may be interposed in the middle); a shaft cylinder configured to contain the writing body in such a manner that the writing body is movable in a front and rear direction with respect to the shaft cylinder; a knock member provided on an outer surface of the shaft cylinder in such a manner that the knock member is movable in a front and rear direction with respect to the shaft cylinder; a projection and retraction rotating member configured to move in a front and rear direction to cause the pen tip to project from or retract within a front end of the shaft cylinder and configured to cause the writing body to rotate, every time the knock member is moved forward; and a cam part provided in the shaft cylinder, the cam part being able to engage or disengage with the projection and retraction rotating member depending on a position of the knock member; wherein a plurality of projection pieces, each of which is projected rearward, are provided at a rear end of the writing body, the plurality of projection pieces being arranged circumferentially apart from each other; a plurality of gaps between the plurality of projection pieces communicate with the air hole; a front portion of the projection and retraction rotating member is provided with a flexible member which is elastically deformable; the flexible member is made of a porous material; and the plurality of projection pieces and the flexible member are configured to circumferentially engage with each other when at least a portion on a rear side of the plurality of projection pieces causes the flexible member to elastically deform and sinks in an inside of the flexible member in such a manner that the writing body rotates in conjunction with a rotation of the projection and retraction rotating member. 
     According to the above structure, due to the flexible member made of a porous material, air permeability can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. In addition, the flexible member can absorb an impact and/or a force applied to the writing body at the time of writing, and thus it is possible to provide a user with a soft and comfortable writing feeling. 
     In addition, according to the sixth aspect of the present invention, a knock type writing tool is provided, the knock type writing tool including: a writing body having a pen tip at a front end thereof, a thermochromic ink in an inside thereof, and an air hole at a rear end thereof, the air hole communicating with a space in which the thermochromic ink is contained (a follower may be interposed in the middle); a shaft cylinder configured to contain the writing body in such a manner that the writing body is movable in a front and rear direction with respect to the shaft cylinder; a knock member provided on an outer surface of the shaft cylinder in such a manner that the knock member is movable in a front and rear direction with respect to the shaft cylinder; a projection and retraction rotating member configured to move in a front and rear direction to cause the pen tip to project from or retract within a front end of the shaft cylinder and configured to cause the writing body to rotate, every time the knock member is moved forward; and a cam part provided in the shaft cylinder, the cam part being able to engage or disengage with the projection and retraction rotating member depending on a position of the knock member; wherein a plurality of projection pieces, each of which is projected rearward, are provided at a rear end of the writing body, the plurality of projection pieces being arranged circumferentially apart from each other; a plurality of gaps between the plurality of projection pieces communicate with the air hole; a front portion of the projection and retraction rotating member is provided with a flexible member which is elastically deformable; and the plurality of projection pieces and the flexible member are configured to circumferentially engage with each other when a portion on a rear side of the plurality of projection pieces causes the flexible member to elastically deform and sinks in an inside of the flexible member in such a manner that the writing body rotates in conjunction with a rotation of the projection and retraction rotating member. 
     According to the above structure, the plurality of gaps between the plurality of projection pieces at a portion thereof that does not sink in the inside of the flexible member can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. 
     Advantageous Effects of Invention 
     According to the first aspect of the present invention, the plurality of gaps between the plurality of projection pieces can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. 
     According to the second aspect of the present invention, the plurality of gaps between the plurality of protrusion parts can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. 
     According to the third aspect of the present invention, due to the flexible member made of a porous material, air permeability can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. In addition, the flexible member can absorb an impact and/or a force applied to the writing body at the time of writing, and thus it is possible to provide a user with a soft and comfortable writing feeling. 
     According to the fourth aspect of the present invention, the connection air hole can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. 
     According to the fifth aspect of the present invention, due to the flexible member made of a porous material, air permeability can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. In addition, the flexible member can absorb an impact and/or a force applied to the writing body at the time of writing, and thus it is possible to provide a user with a soft and comfortable writing feeling. 
     According to the sixth aspect of the present invention, the plurality of gaps between the plurality of projection pieces at a portion thereof that does not sink in the inside of the flexible member can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a longitudinal section view showing a knock type writing tool according to a first embodiment of the present invention, under a state wherein a pen tip is retracted; 
         FIG. 2  is a longitudinal section view showing the knock type writing tool according to the first embodiment of the present invention, under a state wherein the pen tip is projected; 
         FIG. 3  is a longitudinal section view of a writing body shown in  FIG. 1 ; 
         FIG. 4  is a perspective view of a rear end portion of the writing body shown in  FIG. 3 ; 
         FIG. 5  is a plan view of the rear end portion of the writing body shown in  FIG. 3 ; 
         FIG. 6  is a front view of a ferrule of a front shaft shown in  FIG. 1 ; 
         FIG. 7  is a section view taken along line A-A of  FIG. 6 ; 
         FIG. 8  is a front view of a main body of the front shaft shown in  FIG. 1 ; 
         FIG. 9  is a section view taken along line B-B of  FIG. 8 ; 
         FIG. 10  is a longitudinal section view of an intermediate shaft shown in  FIG. 1 ; 
         FIG. 11  is a longitudinal section view of a rear shaft shown in  FIG. 1 ; 
         FIG. 12  is a longitudinal section view of a knock member shown in  FIG. 1 ; 
         FIG. 13  is a perspective view of a projection and retraction rotating member shown in  FIG. 1 ; 
         FIG. 14  is a front view of the projection and retraction rotating member shown in  FIG. 1 ; 
         FIG. 15  is a bottom view of the projection and retraction rotating member shown in  FIG. 14 ; 
         FIG. 16  is a section view taken along line C-C of  FIG. 15 ; 
         FIG. 17  is an explanatory view showing a color change action of a thermochromic ink; 
         FIG. 18  is a schematic view for explaining a principle of a rotating cam mechanism; 
         FIG. 19  is a longitudinal section view showing an abutment state between the projection and retraction rotating member and the rear end of the writing body; 
         FIG. 20  is a perspective view showing the abutment state between the projection and retraction rotating member and the rear end of the writing body; 
         FIG. 21  is a view corresponding to  FIG. 4  with regard to a variant of the first embodiment of the present invention; 
         FIG. 22  is a view corresponding to  FIG. 13  with regard to the variant of the first embodiment of the present invention; 
         FIG. 23  is a view corresponding to  FIG. 14  with regard to the variant of the first embodiment of the present invention; 
         FIG. 24  is a view corresponding to  FIG. 15  with regard to the variant of the first embodiment of the present invention; 
         FIG. 25  is a section view taken along line D-D of  FIG. 24  (a view corresponding to  FIG. 16  with regard to the variant of the first embodiment of the present invention); 
         FIG. 26  is a view corresponding to  FIG. 19  with regard to the variant of the first embodiment of the present invention; 
         FIG. 27  is a view corresponding to  FIG. 20  with regard to the variant of the first embodiment of the present invention; 
         FIG. 28  is a perspective view of a rear end portion of a writing body according to the second embodiment of the present invention; 
         FIG. 29  is a perspective view of a rear end portion of a writing body according to the third embodiment of the present invention; 
         FIG. 30  is a front view showing an abutment state between a rear end of a writing body and a rotating member according to the third and fourth embodiments of the present invention; 
         FIG. 31  is a section view taken along line E-E of  FIG. 30  (a longitudinal section view showing the abutment state between the rear end of the writing body and the rotating member according to the third and fourth embodiments of the present invention); 
         FIG. 32  is a section view of a flexible member according to the third embodiment of the present invention; 
         FIG. 33  is a view corresponding to  FIG. 19  with regard to a variant of the third embodiment of the present invention; 
         FIG. 34  is a perspective view of a rear end portion of a writing body according to the fourth embodiment of the present invention; 
         FIG. 35  is a section view of a flexible member according to the fourth embodiment of the present invention; and 
         FIG. 36  is a view corresponding to  FIG. 19  with regard to a variant of the fourth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     With reference to the attached drawings, we explain embodiments of the present invention hereinafter. 
     Structure of First Embodiment 
     A knock type writing tool according to a first embodiment of the present invention is shown in  FIGS. 1 to 20 . The knock type writing tool  1  according to the present embodiment includes: a shaft cylinder  2 ; a writing body  6  contained in the shaft cylinder  2 ; and a projection and retraction mechanism that causes a pen tip  61  of the writing body  6  to project from and retract within a front end hole  31   a  of the shaft cylinder  2 . 
     (Writing Body) 
     The writing body  6  of the present embodiment is shown in  FIGS. 3 to 5 . With reference to  FIG. 3 , the writing body  6  includes: the pen tip  61 ; an ink containing tube  62  having a front end opening into which the pen tip  61  is press-fitted; an thermochromic ink filled in the ink containing tube  62 ; a follower (for example, high viscosity fluid) filled adjacent to a rear end of the thermochromic ink in such a manner that the follower moves forward as the thermochromic ink is consumed; and a tail plug  63  attached to a rear end opening of the ink containing tube  62 . The tail plug  63  is provided with an air hole  63   a  that opens toward a rear end thereof. The air hole  63   a  can communicate the inside of the ink containing tube  62  and the outside thereof. 
     For example, the pen tip  61  has a structure consisting of only a ball point pen tip made of metal, which holds a ball at a front end thereof in a rotatable manner, or a structure consisting of a combination of such a ball point pen tip and a pen tip holder made of a synthetic resin, which holds a rear outer surface of the ball point pen tip. A spring configured to press forward the ball at the front end is contained in an inside of the pen tip  61 . For example, the spring has a structure wherein a rod part is provided at a front end of a compression coil spring and a front end of the rod part contacts with a rear surface of the ball. When the ball is not used for writing, the ball sealingly contacts with an inwardly-facing front-edge inner surface of a front end of the ball point pen tip. Thereby, ink leakage and ink evaporation from the front end of the pen tip  61  is prevented. 
     In addition, a plurality of projection pieces  63   b  are provided at a rear end of the writing body  6  of the present embodiment, the plurality of projection pieces  63   b  being arranged circumferentially apart from each other. 
     (Tail Plug) 
     The tail plug  63  of the present embodiment is a substantially cylindrical member obtained from a molded body made of a synthetic resin (for example, a polypropylene resin). With reference to  FIG. 3 , the tail plug  63  includes: a smaller diameter part  63   d  located at a front end portion thereof; a larger diameter part  63   e  integrally provided on a rear side of the smaller diameter part  63   d ; a flange part  63   f  integrally provided on a further rear side of the larger diameter part  63   e ; and the plurality of projection pieces  63   b  integrally provided on a further rear side of the flange part  63   f.    
     An annular outward protrusion  63   p  (see  FIG. 19 ) is formed on an outer surface of the larger diameter part  63   e  of the tail plug  63 . When the tail plug  63  is attached on the ink containing tube  62 , the annular outward protrusion  63   p  is annularly press-fitted into an inner surface at a rear end portion of the ink containing tube  62 . Then, a front surface of the flange part  63   f  abuts on a rear end of the ink containing tube  62  (see  FIG. 19 ). 
     An air hole  63   a  is formed through the tail plug  63  in an axial direction (in a front and rear direction). In addition, a plurality of gaps  63   c , each of which has a cutout pattern, are formed between the respective projection pieces  63   b , so that the plurality of gaps  63   c  are radially communicated with the air hole  63   a . Due to the plurality of gaps  63   c  and the air hole  63   a , air can flow between the inside of the ink containing tube  62  and the outside thereof. 
     The present invention is not limited to the manner wherein each of the plurality of projection pieces  63   b  is integrally formed on the rear side of the flange part  63   f . The plurality of projection pieces  63   b  may be formed separately and then joined to the flange part  63   f.    
     (Shaft Cylinder) 
     The shaft cylinder  2  of the present embodiment consists of: a front shaft  3  having a tapered cylindrical shape; an intermediate shaft  4  connected to a rear end portion of the front shaft  3  and having a cylindrical shape; and a rear shaft  5  connected to a rear end portion of the intermediate shaft  4  and having a cylindrical shape. 
     (Front Shaft) 
     The front shaft  3  of the present embodiment consists of: a tapered ferrule  31  made of a metallic material (see  FIGS. 6 and 7 ); a main body  32  connected to a rear end of the ferrule  31 , consisting of a molded body made of a synthetic resin, and having a cylindrical shape (see  FIGS. 8 and 9 ); and a holding part  33  provided on an outer surface of the main body  32  and consisting of an elastic material (see  FIGS. 8 and 9 ). 
     The ferrule  31  and the main body  32  are threadedly engaged. For example, an internal threaded part is formed on an inner surface of a rear end portion of the ferrule  31 , an external threaded part is formed on an outer surface of a front end portion (reduced diameter part) of the main body  32 , and the internal threaded part and the external threaded part are detachably threadedly engaged. 
     The holding part  33  is formed on the outer surface of the main body  32  by a two-color molding, for example, or formed separately and then attached thereon. 
     With reference to  FIG. 9 , a rear end portion of the front shaft  3  (a rear and portion of the main body  32 ) has a reduced diameter, and an external threaded part is formed on an outer surface thereof. The external threaded part is threadedly engaged with an internal threaded part formed on an inner surface of a front end portion of the intermediate shaft  4 . 
     On the other hand, with reference to  FIGS. 6 and 7 , a front end hole  31   a  is formed through a front end portion of the front shaft  3  (a front and portion of the ferrule  31 ) in a front and rear direction. 
     (Intermediate Shaft) 
     With reference to  FIG. 10 , the intermediate shaft  4  of the present embodiment consists of a molded body made of a synthetic resin (for example, a polycarbonate resin). The internal threaded part is formed on the inner surface of the front end portion of the intermediate shaft  4 , and the internal threaded part can be threadedly engaged with the external threaded part at the rear end portion of the front shaft  3 . A rear end portion of the intermediate shaft  4  is integrally provided with a reduced diameter part. 
     The reduced diameter part is provided with a first elongated hole  41  extending in a front and rear direction. A front end of the first elongated hole  41  is closed by a lateral wall of the intermediate shaft  4 . On the other hand, a rear end of the first elongated hole  41  extends to a rear end of the intermediate shaft  4  (i.e., is opened at the rear end of the intermediate shaft  4 ). 
     In addition, an outward protrusion  44  is integrally formed on an outer surface of the reduced diameter part. A step part  42  is integrally formed on an inner surface of the intermediate shaft  4  ahead of the reduced diameter part. 
     A cam part  43  is integrally formed on an inner surface of the intermediate shaft  4  further ahead of the step part  42 . The cam part  43  has a plurality of cam teeth  43   a , each of which has a saw-tooth shape and extends forward, and a plurality of cam grooves  43   b , each of which is formed between the cam teeth  43   a  and extends in a front and rear direction. 
     The front end of the first elongated hole  41  is located slightly ahead of the step part  42  (a rear end of the cam part  43 ) and behind the cam teeth  43   a  of the cam part  43 . 
     (Rear Shaft) 
     With reference to  FIG. 11 , the rear shaft  5  of the present embodiment consists of a substantially cylindrical molded body made of a synthetic resin (for example, a polycarbonate resin). A second elongated hole  51  is formed on a lateral wall of a front end portion of the rear shaft  5 . A rear end of the second elongated hole  51  is closed by the lateral wall of the rear shaft  5 . On the other hand, a front end of the second elongated hole  51  extends to a front end of the rear shaft  5  (i.e., is opened at the front end of the rear shaft  5 ). 
     An attachment hole  52  is formed through a rear end portion of the rear shaft  5  in a front and rear direction. A friction part  53  made of an elastic material is press-fitted into the attachment hole  52 . Thereby, the friction part  53  is fixed to an outer surface of the rear end portion of the rear shaft  5 . In addition, an inward protrusion  54  is integrally formed on an inner surface of the rear shaft  5 . 
     The elastic material for forming the friction part  53  is preferably an elastic synthetic resin (rubber, elastomer), for example, a silicone resin, an SBS resin (styrene-butadiene-styrene copolymer), a SEBS resin (styrene-ethylene-butylene-styrene copolymer), a fluorine-based resin, a chloroprene resin, a nitrile resin, a polyester resin, an ethylene propylene diene rubber (EPDM), or a mixture of two or more rubber elastic materials, or a mixture of a rubber elastic material and a synthetic resin, or the like. 
     Preferably, the elastic synthetic resin constituting the friction part  53  is not an easy-to-wear elastic material (for example, an eraser or the like), but a hard-to-wear elastic material which generates almost no wear debris (for example, eraser shavings) when friction occurs. In addition, the friction part  53  may be provided on an outer surface at a rear end portion of the shaft cylinder  2  in various manners. For example, the friction part  53  made of an elastic material may be provided on the outer surface of the rear end portion of the shaft cylinder  2  or on the outer surface of the rear end portion of the rear shaft  5 , by a press-fitting, an engagement, a threadedly engagement, a fitting, an adhesion, a two-color molding, or the like. Furthermore, the entire shaft cylinder  2  or the entire rear shaft  5  may be integrally made of an elastic material. 
     In addition, a width dimension of the second elongated hole  51  is set to be equal to a width dimension of the first elongated hole  41 . Thus, as will be described below, a knock member  8  can smoothly move in a front and rear direction along a slide hole  21  (=first elongated hole  41 +second elongated hole  51 ). The friction part  53  does not move when the knock member  8  moves in the front and rear direction. 
     (Knock Member) 
     With reference to  FIGS. 1, 2 and 12 , a clip  83  extending in a front and rear direction is fixed to the knock member  8  of the present embodiment. A ball part  83   a  is projected from a back surface of the clip  83 . The knock member  8  consists of: a base part  81  fixed to a rear end portion of the clip  83 ; and a cylindrical shaft part  82  integrally connected to the base part  81  and extending forward form the base part  81 . 
     For example, the knock member  8  is obtained by a molded body made of a synthetic resin (for example, a polycarbonate resin). For example, the clip  83  is obtained from a metallic material (for example, stainless steel). 
     As shown in  FIG. 12 , a plurality of cam teeth  82   a  configured to engage with rear ends of ridges  71  of a projection and retraction rotating member  7  (see  FIGS. 13 to 16 ) are integrally formed at a front end of the shaft part  82 . 
     Herein, how to assemble the knock member  8  is explained. The base part  81  of the knock member  8  is inserted into the first elongated hole  41  of the intermediate shaft  4  from the rear end (opened part) of the first elongated hole  41  while the shaft part  82  of the knock member  8  is inserted into the intermediate shaft  4 . Subsequently. In order that the first elongated hole  41  of the intermediate shaft  4  and the second elongated hole  51  of the rear shaft  5  are radially communicated with each other (radially overlapped), while the inner surface of the rear shaft  5  is fitted onto the outer surface of the reduced diameter part of the intermediate shaft  4 , the base part  81  of the knock member  8  is inserted into the second elongated hole  51  of the rear shaft  5  from the front end (opened part) of the second elongated hole  51 . Thereby, the intermediate shaft  4  and the rear shaft  5  are connected (the outward protrusion  44  of the intermediate shaft  4  and the inward protrusion  54  of the rear shaft  5  are engaged after riding over each other), and the slide hole  21  extending in the front and rear direction is formed by the first elongated hole  41  and the second elongated hole  51 , and the knock member  8  is in a state of projecting radially outward from the slide hole  21 . The knock member  8  is slidable in the front and rear direction along the slide hole  21 . 
     The rear shaft  5 , whose rear end is fixed to the friction part  53 , is connected to the rear end portion of the intermediate shaft  4 , so that the friction part  53  is always fixed to the rear end of the shaft cylinder  2 . In addition, the front shaft  3  and the intermediate shaft  4  are detachably threadedly engaged with each other, so that the writing body  6  is replaceable at any time. 
     (Projection and Retraction Mechanism) 
     The projection and retraction mechanism of the present embodiment is a side-slide type projection and retraction mechanism using a rotating cam mechanism, which includes: the knock member  8  as described above; the projection and retraction rotating member  7  configured to move in the front and rear direction to cause the pen tip  61  to project from or retract within the front end hole  31   a  of the shaft cylinder  2  and configured to cause the writing body  6  (and thus pen tip  61 ) to rotate, every time the knock member  8  is moved forward; the cam part  43  as described above which is provided in the shaft cylinder  2  and is able to engage or disengage with the projection and retraction rotating member  7  depending on a position of the knock member  8 ; and a spring body  9  (for example, a compression coil spring) provided in the shaft cylinder  2  and configured to bias the writing body  6  rearward. 
     The projection and retraction mechanism of the present embodiment is a double-knock type projection and retraction mechanism in which the knock member  8  is operated (pressed) to move (slide) forward both for a pen tip projection operation and for a pet tip retraction operation. 
     (Projection and Retraction Rotating Member) 
     With reference to  FIGS. 13 to 16 , the projection and retraction rotating member  7  of the present embodiment includes: a large diameter front part having a plurality of (for example, four) ridges  71  on an outer surface thereof, each of which extends in a front and rear direction; and a small diameter rear part integrally connected behind the front part and having a substantially cylindrical shape with a bottom. Each of the plurality of ridges  71  is able to engage with the cam teeth  43   a  of the cam part  43  and the cam grooves  43   b  of the cam part  43 . 
     For example, the projection and retraction rotating member  7  is obtained by a molded body made of a synthetic resin (for example, a polyacetal resin). In addition, an outward protrusion  78  is formed on an outer surface of a rear portion of the projection and retraction rotating member  7 . The outward protrusion  78  is engaged with an inward protrusion  88  on an inner surface of the shaft part  82  of the knock member  8  after riding over each other. Thereby, coming off of the knock member  8  and the projection and retraction rotating member  7  is prevented, while the projection and retraction rotating member  7  can have a “play” (a movable amount) in a front and rear direction with respect to the knock member  8  and the knock member  8  and the projection and retraction rotating member  7  can circumferentially rotate to each other. The “play” in the front and rear direction is set to be a small dimension (a height difference (an axial distance) between tops and bottoms of the cam teeth  82   a : see  FIG. 18 ) according to which the ridges  71  of the projection and retraction rotating member  7  and the cam teeth  82   a  can be engaged and disengaged with each other. 
     A plurality of (for example, three) protrusion parts  72 , each of which is protruded forward, are integrally formed on a front surface of the projection and retraction rotating member  7 . The protrusion parts  72  are arranged radially, i.e., at regular intervals in a circumferential direction. 
     In addition, an annular auxiliary protrusion part  74  is also integrally formed on the front surface of the projection and retraction rotating member  7 . A radially inner surface of the annular auxiliary protrusion part  74  is integrally connected to a radially outer end portion of each of the plurality of protrusion parts  72 . 
     Furthermore, an inside surface of the annular auxiliary protrusion part  74  is a tapered auxiliary abutment surface  74   t , and a rear end portion of each projecting piece  63   b  is provided with an auxiliary tapered portion  63   t  on which the auxiliary abutment surface  74   t  is to abut (see  FIG. 4 ). According to this structure, each projecting piece  63   b  can be supported more stably by using the tapered auxiliary abutment surface  74   t . Furthermore, since the auxiliary abutment surface  74   t  and the auxiliary tapered portion  63   t  are tapered, a so-called “centering” effect can be obtained. 
     A plurality of (for example, three) planar abutment parts (regions)  73  (which are in a plane perpendicular to the front and rear direction) are formed on the front surface of the projection and retraction rotating member  7 . Each abutment part  73  is formed (located) radially inner than the annular auxiliary protrusion part  74  and between the plurality of protrusion parts  72 . The rear ends of the projection pieces  63   b  of the tail plug  63  are to abut on the abutment parts  73 . On the other hand, each of the protrusion parts  72  is inserted into a gap between the projection pieces  63   b  of the tail plug  63 . An axial length of the protrusion parts  72  is formed shorter than an axial length of the projection pieces  63   b . Thereby, a portion of each of the plurality of gaps  63   c  is left in front of each of the plurality of protrusion parts  72 , which allows air to be sufficiently taken into the ink containing tube  62  through the air hole  63   a.    
     In addition, a top portion (front end portion) of each of the plurality of protrusion parts  72  has a convex curved surface (for example, a ridge surface having a semicircular cross section (a semi-cylindrical surface)). Thereby, even if the top portions of the protrusion parts  72  contact with the rear ends of the projection pieces  63   b  when the writing body  6  is inserted into the shaft cylinder  2 , rotation of the writing body  6  is smoothly prompted, so that the protrusion parts  72  are smoothly guided between the projection pieces  63   b.    
     In addition, a circumferential width dimension of each protrusion part  72  is set to be smaller than a circumferential width dimension of each gap  63   c  between the projection pieces  63   b  of the tail plug  63 . Thereby, other air passages are formed between lateral surfaces of the protrusion parts  72  and lateral surfaces of the projection pieces  63   b.    
     Herein, in the present embodiment, the front end of the auxiliary abutment surface  74   t  and the front ends of the protrusion parts  72  are located at substantially the same position in an axial direction. However, the front ends of the protrusion parts  72  may be located more forward than the front end of the auxiliary abutment surface  74   t.    
     (Thermochromic Ink) 
     In the present embodiment, it is preferable that the thermochromic Ink is a reversible thermochromic ink. As the reversible thermochromic ink, there are various types, which includes: a heat-decoloring type that decolorizes by heating from a color-developing state; a color-memory holding type that alternately holds a color-developing state or a decoloring state in a specific temperature range; or a heat-coloring type that colorizes (color-develops) by heating from a decoloring state and returns to the decoloring state by cooling from the color-developing state. These types may be used individually or in combination. 
     As a coloring material contained in the reversible thermochromic ink, for example, a reversible thermochromic pigment is preferably used, in which a reversible thermochromic composition is encapsulated in a microcapsule, the reversible thermochromic composition including at least the following three essential components: (a) an electron-donating color-developing organic compound; (b) an electron-accepting compound; and (c) a reaction medium that determines an occurrence temperature of a reaction (color reaction) in which the above two components are chemically linked. 
       FIG. 17  is a graph in which a change of color density depending on a change of temperature is plotted in the case of the thermochromic Ink of the present embodiment. As shown in  FIG. 17 , according to the thermochromic Ink of the present embodiment, the change of color density has different routes between when a temperature thereof is increased from a lower temperature side compared with a color-changing temperature range and when the temperature thereof is decreased from a higher temperature side compared with the color-changing temperature range. 
     In detail, a color-developing state at a low temperature range below a completely color-developing temperature (t 1 ) or a decoloring state at a high temperature range above a completely decoloring temperature (t 4 ) can be maintained as a memory within a specific temperature range (t 2  to t 3 : which is called a substantially two-phase holding temperature range). An ink having such properties is called a color-memory holding type thermochromic ink. 
     In addition, in  FIG. 17 , ΔH indicate a temperature width (a hysteresis width) corresponding to the degree of hysteresis. When a value of ΔH is small, it is difficult to maintain respective states before and after the color change. When the value of ΔH is large, it is easy to maintain respective states before and after the color change. 
     In the case of the thermochromic Ink of the present embodiment, it is assumed that the color change is caused by a friction heat of the friction part  53 . Thus, the completely decoloring temperature (color-changing point on higher temperature side: t 4 ) is set within a range of 25° C. to 95° C. (preferably, 36° C. to 90° C.), and the completely color-developing temperature (color-changing point on lower temperature side: t 1 ) is set within a range of −30° C. to +20° C. (preferably, −30° C. to +10° C.). Thereby, the color-developing state in a normal condition (within a daily living temperature range) can be effectively maintained. Thus, a written trace of the reversible thermochromic Ink can be easily caused to decolorize by the friction heat generated by the friction part  53 . 
     Operation of First Embodiment: Projection and Retraction of Pen Tip 
       FIG. 18  is a schematic view for explaining a principle of the rotating cam mechanism. With reference to  FIG. 18 , from a pen tip retraction state (see  FIG. 18( a ) ), when the knock member  8  is operated (pressed) to move (slide) forward against a rearward biasing force of the spring body  9 , the ridges  71  of the projection and retraction rotating member  7 , which have been engaged with the cam teeth  82   a  at the front end of the shaft part  82  of the knock member  8 , are pressed forward by the cam teeth  82   a . The ridges  71  of the projection and retraction rotating member  7  are moved forward along the cam grooves  43   b.    
     When the ridges  71  of the projection and retraction rotating member  7  are moved forward, the projection and retraction rotating member  7  presses the writing body  6  forward according to abutment relationship between the abutment parts  73  and the projection pieces  63   b  of the tail plug  63 . Then, the pen tip  61  is projected outward from the front end hole  31   a.    
     At the deepest position of the knocking operation of the knock member  8 , the rear ends of the ridges  71  get out of the cam grooves  43   b , and move (fall) into the bottoms of the cam teeth  82   a  of the knock member  8 . Thereby, the projection and retraction rotating member  7  slightly rotates circumferentially (see  FIG. 18( b ) ). 
     Subsequently, when the knock member  8  is returned to an original position thereof by the rearward biasing force of the spring body  9 , the rear ends of the ridges  71  move (fall) into bottoms of the cam teeth  43   a  of the cam part  43 . Thereby, the projection and retraction rotating member  7  further rotates circumferentially (see  FIG. 18( c ) ). In addition, the rear ends of the ridges  71  stay in the bottoms of the cam teeth  43   a  of the cam part  43 , so that a pen tip projection state is maintained. 
     Next, from the pen tip projection state (see  FIG. 18( c ) ), when the knock member  8  is operated (pressed) to move (slide) forward, the ridges  71  of the projection and retraction rotating member  7  are again engaged with the cam teeth  82   a  at the front end of the shaft part  82  of the knock member  8 , and are pressed forward by the cam teeth  82   a.    
     At the deepest position of the knocking operation of the knock member  8 , the rear ends of the ridges  71  ride over tops (teeth) of the cam teeth  43   a  of the cam part  43 , and move (fall) into the bottoms of the cam teeth  82   a  of the knock member  8 . Thereby, the projection and retraction rotating member  7  slightly rotates circumferentially (see  FIG. 18( d ) ). Subsequently, when the knock member  8  is returned to the original position thereof by the rearward biasing force of the spring body  9 , the ridges  71  are guided into the cam grooves  43   a , so that the pen tip retraction state is maintained (see  FIG. 18( a ) ). 
     Operation of First Embodiment: Rotation of Pen Tip 
     The abutment state between the projection pieces  63   b  of the tail plug  63  and the abutment parts  73  of the projection and retraction rotating member  7  is shown in  FIGS. 19 and 20 . 
     The abutment parts  73  on the front surface of the projection and retraction rotating member  7  abut on the projection pieces  63   b  of the tail plug  63  at the rear end of the writing body  6 , while the plurality of protrusion parts  72  on the front surface of the projection and retraction rotating member  7  are inserted into the plurality of gaps  63   c  between the projection pieces  63   b  of the tail plug  63  at the rear end of the writing body  6  so that the protrusion parts  72  are circumferentially engaged with the projection pieces  63   b . Thereby, when the projection and retraction rotating member  7  rotates circumferentially, the writing body  6  also rotates circumferentially. 
     Herein, both engagement surfaces  63   g  of the projection pieces  63   b  of the tail plug  63  (the lateral surfaces of the projection pieces  63   b  in the circumferential direction) and engagement surfaces  72   a  of the protrusion parts  72  of the projection and retraction rotating member  7  (the lateral surfaces of the protrusion parts  72  in the circumferential direction) extend substantially in parallel to the front and rear direction (axial direction). Thus, the circumferential engagement of the projection and retraction rotating member  7  with the writing body  6  can be obtained more surely. 
     The circumferential width dimension of each gap  63   c  between the projection pieces  63   b  is set to be larger than the circumferential width dimension of each protrusion part  72 . For example, the circumferential width dimension of each gap  63   c  between the projection pieces  63   b  may be set to be 1.5 mm, and the circumferential width dimension of each protrusion part  72  is set to be 0.7 mm. 
     In addition, a protrusion length of the protrusion parts  72  in the axial direction (an axial length of the protrusion parts  72  protruding forward from the abutment parts  73 ) is preferably not less than 0.5 mm and not greater than 2.5 mm. In addition, a projection length of the projection pieces  63   b  in the axial direction (an axial length from the rear ends (distal ends) of the projection pieces  63   b  to the proximal ends of the projection pieces  63   b ) is preferably not less than 2 mm and not greater than 5 mm. When these conditions are satisfied, the protrusion parts  72  and the projection pieces  63   b  can be engaged more surely. 
     Operation of First Embodiment: Appropriate Taking of Air 
     On the other hand, it is preferable that the difference between the projection length of the projection pieces  63   b  in the axial direction and the protrusion length of the protrusion parts  72  in the axial direction is not less than 1 mm. In this case, a portion of each of the plurality of gaps  63   c  is left in front of each of the plurality of protrusion parts  72 , which allows air to be sufficiently taken into the ink containing tube  62  through the air hole  63   a . Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip  61 , so that the thermochromic ink can be smoothly discharged from the pen tip  61 . 
     Effects of First Embodiment 
     The knock type writing tool  1  of the present embodiment as described above includes: the writing body  6  having the pen tip  61  at the front end thereof, the thermochromic ink in the inside thereof, and the air hole  63   a  at the rear end thereof, the air hole  63   a  communicating with the space in which the thermochromic ink is contained; the shaft cylinder  2  configured to contain the writing body  6  in such a manner that the writing body  6  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the knock member  8  provided on the outer surface of the shaft cylinder  2  in such a manner that the knock member  8  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the projection and retraction rotating member  7  configured to move in the front and rear direction to cause the pen tip  61  to project from or retract within the front end of the shaft cylinder  2  and configured to cause the writing body  6  to rotate, every time the knock member  8  is moved forward; and the cam part  43  provided in the shaft cylinder  2 , the cam part  43  being able to engage or disengage with the projection and retraction rotating member  7  depending on the position of the knock member  8 ; wherein the plurality of projection pieces  63   b , each of which is projected rearward, are provided at the rear end of the writing body  6 , the plurality of projection pieces  63   b  being arranged circumferentially apart from each other; the plurality of gaps  63   c  between the plurality of projection pieces  63   b  communicate with the air hole  63   a ; the front portion of the projection and retraction rotating member  7  is provided with the plurality of abutment parts  73 , on which the respective rear ends of the plurality of projection pieces  63   b  are to abut, and the plurality of protrusion parts  72 , each of which is protruded more forward than the plurality of abutment parts  73 ; the plurality of protrusion parts  72  are respectively inserted in the plurality of gaps  63   c  between the plurality of projection pieces  63   b ; and a portion of each of the plurality of gaps  63   c  is left in front of each of the plurality of protrusion parts  72   b.    
     According to this feature, the plurality of gaps  63   c  between the plurality of projection pieces  63   b  can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip  61 , so that the thermochromic ink can be smoothly discharged from the pen tip  61 . 
     In addition, according to the knock type writing tool  1  of the present embodiment, the shaft cylinder  2  is configured to contain the writing body  6  in such a manner that the writing body  6  is movable in the front and rear direction with respect to the shaft cylinder  2  under a state in which the writing body  6  is biased rearward; the projection and retraction rotating member  7  is configured to move in the front and rear direction, in conjunction with the movement of the knock member  8  in the front and rear direction; and the projection and retraction rotating member  7  is configured to move forward together with the knock member  8  and to cause the pen tip  61  to project from or retract within the front end of the shaft cylinder  2 , every time the knock member  8  is moved forward. 
     According to this feature, the knock member  8  is also affected by a force for biasing the writing body  6  rearward. Thus, the knock member  8  can be automatically returned, which can provide a good operation feeling and a good operability. 
     In addition, according to the knock type writing tool  1  of the present embodiment, the plurality of projection pieces  63   b  and the plurality of protrusion parts  72  are configured to circumferentially engage with each other in such a manner that the writing body  6  rotates in conjunction with the rotation of the projection and retraction rotating member  7 . 
     According to this feature, although the structure is simple, the circumferential engagement of the projection and retraction rotating member  7  with the writing body  6  can be surely obtained. 
     In addition, according to the knock type writing tool  1  of the present embodiment, the engagement surfaces  63   g  of the plurality of projection pieces  63   b  against the plurality of protrusion parts  72  and the engagement surfaces  72   a  of the plurality of protrusion parts  72  against the plurality of projection pieces  63   b  extend substantially in parallel in the front and rear direction. 
     According to this feature, the respective engagement surfaces are engaged more efficiently, so that the circumferential engagement of the projection and retraction rotating member  7  with the writing body  6  can be obtained more surely. 
     In addition, according to the knock type writing tool  1  of the present embodiment, the front portion of the projection and retraction rotating member  7  is connected to the annular auxiliary protrusion part  74  provided integrally with the plurality of protrusion parts  72 . 
     According to this feature, the plurality of protrusion parts  72  are effectively reinforced, which can prevent breakage of the plurality of protrusion parts  72  or the like. 
     In addition, according to the knock type writing tool  1  of the present embodiment, the inside surface of the annular auxiliary protrusion part  74  is the tapered auxiliary abutment surface  74   t , and the rear end portion of the writing body  6  (the rear end portion of each projection piece  63   b ) is provided with the auxiliary tapered portion  63   t  on which the auxiliary abutment surface  74   t  is to abut. 
     According to this feature, the rear end portion of the writing body  6  can be supported more stably by using the tapered auxiliary abutment surface  74   t . Furthermore, since the auxiliary abutment surface  74   t  and the auxiliary tapered portion  63   t  are tapered, a so-called “centering” effect can be obtained. 
     In addition, according to the knock type writing tool  1  of the present embodiment, the plurality of projection pieces  63   b  are formed of polypropylene resin or polyethylene resin. 
     According to this feature, the plurality of projection pieces  63   b  can have sufficient strength, so that bending or buckling thereof can be effectively prevented. Thus, a stable rotation of the writing body  6  can be obtained in conjunction with the rotation of the projection and retraction rotating member  7 . 
     As a variation of the present embodiment, the knock member  8  may be configured in such a manner that the knock member  8  is projected rearward from the rear end of the shaft cylinder  2 . 
     Variant of First Embodiment 
     In the first embodiment, the projection length of the projection pieces  63   b  in the axial direction is longer than the protrusion length of the protrusion parts  72  in the axial direction. However, the protrusion length of the protrusion parts  72  in the axial direction may be longer than the projection length of the projection pieces  63   b  in the axial direction. In the latter case, a gap  72   c  is left behind each of the projection pieces  63   b , which ensures that air is sufficiently taken into the ink containing tube  62  through the air hole  63   a.    
     Such a variant is shown in  FIGS. 21 to 27 .  FIG. 21  is a view corresponding to  FIG. 4  with regard to the variant.  FIGS. 22 to 25  are views corresponding to  FIGS. 13 to 16  with regard to the variant.  FIGS. 26 and 27  are views corresponding to  FIGS. 19 and 20  with regard to the variant. 
     As shown in  FIGS. 21 to 27 , in the same way as the knock type writing tool  1  of the first embodiment, the knock type writing tool of the present variation also includes: the writing body  6  having the pen tip  61  at the front end thereof, the thermochromic ink in the inside thereof, and the air hole  63   a  at the rear end thereof, the air hole  63   a  communicating with the space in which the thermochromic ink is contained; the shaft cylinder  2  configured to contain the writing body  6  in such a manner that the writing body  6  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the knock member  8  provided on the outer surface of the shaft cylinder  2  in such a manner that the knock member  8  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the projection and retraction rotating member  7  configured to move in the front and rear direction to cause the pen tip  61  to project from or retract within the front end of the shaft cylinder  2  and configured to cause the writing body  6  to rotate, every time the knock member  8  is moved forward; and the cam part  43  provided in the shaft cylinder  2 , the cam part  43  being able to engage or disengage with the projection and retraction rotating member  7  depending on the position of the knock member  8 ; wherein the plurality of protrusion parts  72 , each of which is protruded forward, are provided at the front portion of the projection and retraction rotating member  7 , the plurality of protrusion parts  72  being arranged circumferentially apart from each other. 
     On the other hand, in the present variant, the rear end of the writing body  6  is provided with a plurality of abutment parts  63   v , on which respective front ends of the plurality of protrusion parts  72  are to abut, and the plurality of projection pieces  63   b , each of which is projected more rearward than the plurality of abutment parts  63   v . In addition, the plurality of projection pieces  63   b  are respectively inserted in the plurality of gaps  72   c  between the plurality of protrusion parts  72 , and a portion of each of the plurality of gaps  72   c  is left behind each of the plurality of projection pieces  63   b.    
     The other structure of the present variant is substantially the same as that of the first embodiment explained with reference to  FIGS. 1 to 20 . In  FIGS. 21 to 27 , the same parts as those of the first embodiment are shown by the same reference numerals, and detailed explanation thereof is omitted. 
     According to the present variant, the plurality of gaps  72   c  between the plurality of protrusion parts  72  can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip, so that the thermochromic ink can be smoothly discharged from the pen tip. 
     In the variant as well, it is preferable that the difference between the projection length of the projection pieces  63   b  in the axial direction and the protrusion length of the protrusion parts  72  in the axial direction is not less than 1 mm. 
     Structure of Second Embodiment 
     Subsequently,  FIG. 28  is a perspective view of a rear end portion of a writing body  6  according to the second embodiment of the present invention. 
     As shown in  FIG. 28 , the writing body of the present embodiment does not have a tail plug, and a plurality of projection pieces  63   b  are integrally formed at a rear end of an ink containing tube  62 . In addition, at the rear end of the ink containing tube  62 , an air hole  63   a  is opened, through which air can flow between an inside of the ink containing tube  62  and an outside thereof. 
     A plurality of gaps  63   c , each of which has a cutout pattern, are formed between the respective projection pieces  63   b , so that the plurality of gaps  63   c  are radially communicated with the air hole  63   a . Due to the plurality of gaps  63   c  and the air hole  63   a , air can flow between the inside of the ink containing tube  62  and the outside thereof. 
     The other structure of the present embodiment is substantially the same as that of the first embodiment explained with reference to  FIGS. 1 to 20 . In  FIG. 28 , the same parts as those of the first embodiment are shown by the same reference numerals, and detailed explanation thereof is omitted. 
     According to the present embodiment, since the plurality of projection pieces  63  are integrally formed at the rear end of the ink containing tube  62 , a rotation of the writing body  6  can be more surely obtained in conjunction with the rotation of the projection and retraction rotating member  7 . In addition, it is possible to reduce the number of components, so that productivity can be improved. 
     A preferable method for integrally forming the plurality of projection pieces  63   b  and the ink containing tube  62  is a cutting process of the rear end portion of the ink containing tube  62 . According to the cutting process, the shape of each projection piece  63   b  can be formed more surely, so that a rotation of the writing body  6  can be more surely obtained in conjunction with the rotation of the projection and retraction rotating member  7 . 
     Another preferable method for integrally forming the plurality of projection pieces  63  and the ink containing tube  62  is an injection molding process of the ink containing tube  62 . According to the injection molding process as well, the shape of each projection piece  63   b  can be formed more surely, so that a rotation of the writing body  6  can be more surely obtained in conjunction with the rotation of the projection and retraction rotating member  7 . In addition, the projection pieces  63   b  can be integrally formed at the rear end pf the ink containing tube  62  at the same time as the injection molding process of the ink containing tube  62 , so that productivity can be improved. 
     Structure of Third Embodiment 
     Subsequently,  FIG. 29  is a perspective view of a rear end portion of a writing body  6  according to the third embodiment of the present invention,  FIG. 30  is a front view showing an abutment state between the projection and retraction rotating member  7  and the rear end of the writing body  6  of the third embodiment, and  FIG. 31  is a section view taken along line E-E of  FIG. 30 . 
     In the same way as the knock type writing tool  1  of the first embodiment, the knock type writing tool of the present embodiment also includes: the writing body  6  having the pen tip  61  at the front end thereof, the thermochromic ink in the inside thereof, and the air hole  63   a  at the rear end thereof, the air hole  63   a  communicating with the space in which the thermochromic ink is contained; the shaft cylinder  2  configured to contain the writing body  6  in such a manner that the writing body  6  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the knock member  8  provided on the outer surface of the shaft cylinder  2  in such a manner that the knock member  8  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the projection and retraction rotating member  7  configured to move in the front and rear direction to cause the pen tip  61  to project from or retract within the front end of the shaft cylinder  2  and configured to cause the writing body  6  to rotate, every time the knock member  8  is moved forward; and the cam part  43  provided in the shaft cylinder  2 , the cam part  43  being able to engage or disengage with the projection and retraction rotating member  7  depending on the position of the knock member  8 ; wherein the plurality of protrusion parts  72 , each of which is protruded forward, are provided at the front portion of the projection and retraction rotating member  7 . 
     On the other hand, in the present embodiment, a flexible member  64 , which is elastically deformable, is provided at the rear end of the writing body  6  (so as to cover the air hole  63   a ). The flexible member  64  is a cylindrical member made of a porous material (for example, a porous body such as a sponge). The writing body  6  does not have a tail plug  63  nor a projection piece  63   b.    
     The flexible member  64  is porous, so that the flexible member  64  provides communication holes  64   a  (a large number of small communication holes  64   a ). In addition, as shown in  FIG. 32 , the flexible member  64  has: a thin diameter part  64   d  on a front side thereof; a small diameter part  64   e  in the middle thereof, and a large diameter part  64   f  on a rear side thereof. Under a state in which the flexible member  64  is attached to the ink containing tube  62 , an outer surface of the small diameter part  64   e  is press-fitted into an inner surface of the rear end portion of the ink containing tube  62  and a front surface of the large diameter part  64   f  abuts on the rear end of the ink containing tube  62 . Due to the communication holes  64   a , air can flow between the inside of the ink containing tube  62  and the outside thereof. 
     The plurality of protrusion parts  72  and the flexible member  64  are configured to circumferentially engage with each other when the plurality of protrusion parts  72  cause the flexible member  64  to elastically deform with an aid of the biasing force of the spring body  9  and sink in an inside of the flexible member  64  (see  FIGS. 30 and 31 ) in such a manner that the writing body  6  rotates in conjunction with a rotation of the projection and retraction rotating member  7 . 
     The other structure of the present embodiment is substantially the same as that of the first embodiment explained with reference to  FIGS. 1 to 20 . In  FIGS. 29 to 31 , the same parts as those of the first embodiment are shown by the same reference numerals, and detailed explanation thereof is omitted. 
     According to the present embodiment, due to the flexible member  64  made of a porous material, air permeability can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip  61 , so that the thermochromic ink can be smoothly discharged from the pen tip  61 . In addition, the flexible member  64  can absorb an impact and/or a force applied to the writing body  6  at the time of writing, and thus it is possible to provide a user with a soft and comfortable writing feeling. 
     In addition, according to the present embodiment, due to the elastically deforming of the flexible member  64 , the flexible member  64  and the protrusion parts  72  are circumferentially engaged with each other more surely, and thus a rotation of the writing body  6  can be more surely obtained in conjunction with the rotation of the projection and retraction rotating member  7 . 
     Furthermore, according to the present embodiment, for example when the writing body  6  falls down with the flexible member  64  facing downward, the elasticity of the flexible member  64  can protect the writing body  6  from an impact of the falling down. 
     In the present embodiment, the flexible member  64  may be integrally formed at the rear end of the ink containing tube  62 . As a method for integrally forming the flexible member  64  and the ink containing tube  62 , an adhesion, a welding, an insert molding or the like can be cited. 
     Variant of Third Embodiment 
     In the third embodiment, the flexible member  64  is provided at the rear end portion of the writing body  6  while the same projection and retraction rotating member  7  as the first embodiment is used. However, as shown in  FIG. 33 , a flexible member may be provided at a front portion of a projection and retraction rotating member  7 ′ while substantially the same projection pieces  63   b ′ as the first embodiment are used. 
     In the same way as the knock type writing tool  1  of the first embodiment, the knock type writing tool of the present variant also includes: the writing body  6  having the pen tip  61  at the front end thereof, the thermochromic ink in the inside thereof, and the air hole  63   a  at the rear end thereof, the air hole  63   a ′ communicating with the space in which the thermochromic ink is contained; the shaft cylinder  2  configured to contain the writing body  6  in such a manner that the writing body  6  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the knock member  8  provided on the outer surface of the shaft cylinder  2  in such a manner that the knock member  8  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the projection and retraction rotating member  7 ′ configured to move in the front and rear direction to cause the pen tip  61  to project from or retract within the front end of the shaft cylinder  2  and configured to cause the writing body  6  to rotate, every time the knock member  8  is moved forward; and the cam part  43  provided in the shaft cylinder  2 , the cam part  43  being able to engage or disengage with the projection and retraction rotating member  7 ′ depending on the position of the knock member  8 . 
     In the present variant, in substantially the same way as the first embodiment, the plurality of projection pieces  63   b ′, each of which is projected rearward, are provided at the rear end of the writing body  6  more rearward than a flange part  63   f ′, the plurality of projection pieces  63   b ′ being arranged circumferentially apart from each other. The plurality of gaps  63   c ′ between the plurality of projection pieces  63   b ′ communicate with the air hole  63   a ′. However, the length of the projection pieces  63   b ′ is shorter than the length of the projection pieces  63   b  in the first embodiment (see  FIG. 19 ), and thus the length of the gaps  63   c ′ is correspondingly shorter than the length of the gaps  63   c  in the first embodiment (see  FIG. 19 ) (Instead, the length of the flange part  63   f ′ is longer than the length of the flange part  63   f  in the first embodiment (see  FIG. 19 )). 
     On the other hand, in the present variant, a flexible member  79 ′, which is elastically deformable, is provided at the front portion of the projection and retraction rotating member  7 ′ in such a manner that the flexible member  79 ′ is contained in an annular protrusion part  74 ′. For example, the flexible member  79 ′ is made of a porous material (for example, a porous body such as a sponge). 
     The plurality of projection pieces  63   b ′ and the flexible member  79 ′ are configured to circumferentially engage with each other when at least a portion (the entire portion in the example shown in  FIG. 33 ) on a rear side of the plurality of projection pieces  63   b ′ causes the flexible member  79 ′ to elastically deform with an aid of the biasing force of the spring body  9  and sinks in an inside of the flexible member  79 ′ in such a manner that the writing body  6  rotates in conjunction with a rotation of the projection and retraction rotating member  7 ′. 
     According to the present variant as well, due to the flexible member  79 ′ made of a porous material, air permeability can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip  61 , so that the thermochromic ink can be smoothly discharged from the pen tip  61 . In addition, the flexible member  79 ′ can absorb an impact and/or a force applied to the writing body  6  at the time of writing, and thus it is possible to provide a user with a soft and comfortable writing feeling. 
     In addition, according to the present variant as well, due to the elastically deforming of the flexible member  79 ′, the flexible member  79 ′ and the projection pieces  63   b ′ are circumferentially engaged with each other more surely, and thus a rotation of the writing body  6  can be more surely obtained in conjunction with the rotation of the projection and retraction rotating member  7 ′. 
     Furthermore, according to the present variant as well, for example when the writing body  6  falls down with the flexible member  79 ′ facing downward, the elasticity of the flexible member  79 ′ can protect the writing body  6  from an impact of the falling down. 
     Structure of Fourth Embodiment 
     Subsequently,  FIG. 34  is a perspective view of a rear end portion of a writing body according to the fourth embodiment of the present invention. 
     In the same way as the knock type writing tool  1  of the first embodiment, the knock type writing tool of the present embodiment also includes: the writing body  6  having the pen tip  61  at the front end thereof, the thermochromic ink in the inside thereof, and the air hole  63   a  at the rear end thereof, the air hole  63   a  communicating with the space in which the thermochromic ink is contained; the shaft cylinder  2  configured to contain the writing body  6  in such a manner that the writing body  6  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the knock member  8  provided on the outer surface of the shaft cylinder  2  in such a manner that the knock member  8  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the projection and retraction rotating member  7  configured to move in the front and rear direction to cause the pen tip  61  to project from or retract within the front end of the shaft cylinder  2  and configured to cause the writing body  6  to rotate, every time the knock member  8  is moved forward; and the cam part  43  provided in the shaft cylinder  2 , the cam part  43  being able to engage or disengage with the projection and retraction rotating member  7  depending on the position of the knock member  8 ; wherein the plurality of protrusion parts  72 , each of which is protruded forward, are provided at the front portion of the projection and retraction rotating member  7 . 
     On the other hand, in the present embodiment, a flexible member  64 ′, which is elastically deformable, is provided at the rear end of the writing body  6  (so as to cover the air hole  63   a ). The flexible member  64 ′ is a cylindrical body formed by a molded body of a thermoplastic elastomer. The flexible member  64 ′ has a communication hole  64   a ′ (connection air hole) opened at a rear end thereof and extending in a front and rear direction (axial direction). The writing body  6  does not have a tail plug  63  nor a projection piece  63   b.    
     The communication hole  64   a ′ of the flexible member  64 ′ communicates with the air hole  63   a . In addition, as shown in  FIG. 35 , the flexible member  64 ′ also has: a thin diameter part  64   d ′ on a front side thereof; a small diameter part  64   e ′ in the middle thereof, and a large diameter part  64   f ′ on a rear side thereof. Under a state in which the flexible member  64 ′ is attached to the ink containing tube  62 , an outer surface of the small diameter part  64   e ′ is press-fitted into an inner surface of the rear end portion of the ink containing tube  62  and a front surface of the large diameter part  64   f ′ abuts on the rear end of the ink containing tube  62 . Due to the communication holes  64   a ′, aft can flow between the inside of the ink containing tube  62  and the outside thereof. 
     The plurality of protrusion parts  72  and the flexible member  64 ′ are configured to circumferentially engage with each other when the plurality of protrusion parts  72  cause the flexible member  64 ′ to elastically deform with an aid of the biasing force of the spring body  9  and sink in an inside of the flexible member  64 ′ (see  FIGS. 30 and 31 ) in such a manner that the writing body  6  rotates in conjunction with a rotation of the projection and retraction rotating member  7 . 
     The other structure of the present embodiment is substantially the same as that of the first embodiment explained with reference to  FIGS. 1 to 20 . In  FIG. 34 , the same parts as those of the first embodiment are shown by the same reference numerals, and detailed explanation thereof is omitted. 
     According to the present embodiment, due to the communication hole  64   a ′ of the flexible member  64 ′, air permeability can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip  61 , so that the thermochromic ink can be smoothly discharged from the pen tip  61 . 
     Even when the abutment parts  73  of the projection and retraction rotating member  7  abut on a rear end surface of the flexible member  64 ′, a gap is formed between each protrusion part  72  and the flexible member  64 ′, and the gap can function as an air communication hole. Thereby, aft permeability can be secured sufficiently. 
     In addition to the communication hole  64   a ′ or instead of the communication hole  64   a ′, a lateral surface of the flexible member  64 ′ may be provided with a lateral hole configured to communicate with the air hole  63   a . The lateral hole may function as an aft communication hole (connection aft hole). 
     In addition, the flexible member  64 ′ can absorb an impact and/or a force applied to the writing body  6  at the time of writing, and thus it is possible to provide a user with a soft and comfortable writing feeling. 
     In addition, according to the present embodiment, due to the elastically deforming of the flexible member  64 ′, the flexible member  64 ″ and the protrusion parts  72  are circumferentially engaged with each other more surely, and thus a rotation of the writing body  6  can be more surely obtained in conjunction with the rotation of the projection and retraction rotating member  7 . 
     Furthermore, according to the present embodiment, for example when the writing body  6  falls down with the flexible member  64 ′ facing downward, the elasticity of the flexible member  64 ′ can protect the writing body  6  from an impact of the falling down. 
     In the present embodiment, the flexible member  64 ′ may be integrally formed at the rear end of the ink containing tube  62 . As a method for integrally forming the flexible member  64 ′ and the ink containing tube  62 , an adhesion, a welding, an insert molding or the like can be cited. 
     Variant of Fourth Embodiment 
     In the fourth embodiment, the flexible member  64 ′ is provided at the rear end portion of the writing body  6  while the same projection and retraction rotating member  7  as the first embodiment is used. However, as shown in  FIG. 36 , a flexible member may be provided at a front portion of a projection and retraction rotating member  7 ″ while substantially the same projection pieces  63   b ″ as the first embodiment are used. 
     In the same way as the knock type writing tool  1  of the first embodiment, the knock type writing tool of the present variant also includes: the writing body  6  having the pen tip  61  at the front end thereof, the thermochromic ink in the inside thereof, and the air hole  63   a  at the rear end thereof, the air hole  63   a ″ communicating with the space in which the thermochromic ink is contained; the shaft cylinder  2  configured to contain the writing body  6  in such a manner that the writing body  6  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the knock member  8  provided on the outer surface of the shaft cylinder  2  in such a manner that the knock member  8  is movable in the front and rear direction with respect to the shaft cylinder  2 ; the projection and retraction rotating member  7 ″ configured to move in the front and rear direction to cause the pen tip  61  to project from or retract within the front end of the shaft cylinder  2  and configured to cause the writing body  6  to rotate, every time the knock member  8  is moved forward; and the cam part  43  provided in the shaft cylinder  2 , the cam part  43  being able to engage or disengage with the projection and retraction rotating member  7 ″ depending on the position of the knock member  8 . 
     In the present variant, in substantially the same way as the first embodiment, the plurality of projection pieces  63   b ″, each of which is projected rearward, are provided at the rear end of the writing body  6  more rearward than a flange part  63   f ″, the plurality of projection pieces  63   b ″ being arranged circumferentially apart from each other. The plurality of gaps  63   c ″ between the plurality of projection pieces  63   b ″ communicate with the air hole  63   a″.    
     On the other hand, in the present variant, a flexible member  79 ″, which is elastically deformable, is provided at the front portion of the projection and retraction rotating member  7 ″ in such a manner that the flexible member  79 ″ is contained in an annular protrusion part  74 ″. For example, the flexible member  79 ″ is formed by a molded body of a thermoplastic elastomer. 
     The plurality of projection pieces  63   b ″ and the flexible member  79 ″ are configured to circumferentially engage with each other when a portion on a rear side of the plurality of projection pieces  63   b ″ causes the flexible member  79 ″ to elastically deform with an aid of the biasing force of the spring body  9  and sinks in an inside of the flexible member  79 ″ in such a manner that the writing body  6  rotates in conjunction with a rotation of the projection and retraction rotating member  7 ″. 
     According to the present variant, due to the gaps between the plurality of projection pieces  63   b ″ on the front side (on the proximal side), air permeability can be secured sufficiently. Thus, an appropriate amount of air can be taken in depending on an amount of the thermochromic ink discharged from the pen tip  61 , so that the thermochromic ink can be smoothly discharged from the pen tip  61 . In addition, the flexible member  79 ″ can absorb an impact and/or a force applied to the writing body  6  at the time of writing, and thus it is possible to provide a user with a soft and comfortable writing feeling. 
     In addition, according to the present variant as well, due to the elastically deforming of the flexible member  79 ″, the flexible member  79 ″ and the projection pieces  63   b ″ are circumferentially engaged with each other more surely, and thus a rotation of the writing body  6  can be more surely obtained in conjunction with the rotation of the projection and retraction rotating member  7 ″. 
     Furthermore, according to the present variant as well, for example when the writing body  6  falls down with the flexible member  79 ″ facing downward, the elasticity of the flexible member  79 ″ can protect the writing body  6  from an impact of the falling down. 
     EXPLANATION OF SIGN 
     
         
           1  knock type writing tool 
           2  shaft cylinder 
           3  front shaft 
           4  intermediate shaft 
           5  rear shaft 
           6  writing body 
           7  projection and retraction rotating member 
           7 ′ projection and retraction rotating member 
           7 ″ projection and retraction rotating member 
           8  knock member 
           9  spring body 
           21  slide hole 
           31  ferrule 
           31   a  front end hole 
           32  main body 
           33  holding part 
           41  first elongated hole 
           42  step part 
           43  cam part 
           43   a  cam tooth 
           43   b  cam groove 
           44  outward protrusion 
           51  second elongated hole 
           52  attachment hole 
           53  friction part 
           54  inward protrusion 
           61  pen tip 
           62  ink containing tube 
           63  tail plug 
           63 ′ tail plug 
           63   a  air hole 
           63   a ′ air hole 
           63   a ″ air hole 
           63   b  projection piece 
           63   b ′ projection piece 
           63   b ″ projection piece 
           63   c  gap 
           63   c ′ gap 
           63   c ″ gap 
           63   d  smaller diameter part 
           63   e  larger diameter part 
           63   f  flange part 
           63   f ′ flange part 
           63   f ″ flange part 
           63   g  engagement surface 
           63   p  outward protrusion 
           63   t  auxiliary tapered portion 
           63   v  abutment part 
           64  flexible member 
           64 ′ flexible member 
           64   a  communication hole 
           64   a ′ communication hole 
           64   d  thin diameter part 
           64   d ′ thin diameter part 
           64   e  small diameter part 
           64   e ′ small diameter part 
           64   f  large diameter part 
           64   f ′ large diameter part 
           71  ridge 
           72  protrusion part 
           72   a  engagement surface 
           72   c  gap 
           73  abutment part 
           74  annular auxiliary protrusion part 
           74 ′ annular protrusion part 
           74 ″ annular protrusion part 
           74   t  auxiliary abutment surface 
           78  outward protrusion 
           79 ′ flexible member 
           79 ″ flexible member 
           81  base part 
           82  shaft part 
           82   a  cam tooth 
           83  clip 
           83   a  ball part 
           88  inward protrusion