the drawings illustrate a drainage plug ( a ) installed in a reservoir tank ( b ) of a basin . the drainage plug ( a ) is constructed such that a guide cylinder ( 3 ) is arranged in a drainage port ( 2 ) of a main body ( 1 ) of the drainage plug that is fitted in a bottom part of the reservoir tank ( b ). a supporting shaft ( 5 ) supporting a plug cap ( 4 ) is fitted in a guide cylinder ( 3 ) in such a way that it may be advanced or retracted in its upward or downward direction , and the supporting shaft ( 5 ) is engaged with a thrust lock mechanism ( 6 ) arranged in the guide cylinder ( 3 ). the main body ( 1 ) of the drainage plug is a cylindrical body formed with a concave part ( 1a ) and a flange part ( 1b ) at its upper edge . the main body is inserted from above to a fixing port in the reservoir tank ( b ), and a drainage pipe ( c ) is connected to the lower end thereof . fixing pieces ( 1c ) and ( 1c ) project toward the center of the drainage port ( 2 ) from an inner circumferential surface of the drainage port ( 2 ) of the main body ( 1 ). the guide cylinder ( 3 ) is bolted at ( 7 ) to the fixing pieces ( 1c ) and ( 1c ) so that the guide cylinder ( 3 ) is vertically held in the central part of the drainage port ( 2 ). the guide cylinder ( 3 ) is comprised of a main body ( 3a ) of the guide cylinder and a cap ( 3b that is fixed to the fixing pieces ( 1c ) and ( 1c ). the supporting shaft ( 5 ) is inserted in the central part of the cylinder so that it may slide in upward or downward directions . packings ( 8 ) and ( 8 &# 39 ;) are fitted in upper and lower fitting portions at both ends forming a sealed condition . the supporting shaft ( 5 ) and thrust lock mechanism ( 6 ) are assembled within the cylinder ( 3 ) so as to connect the main body ( 3a ) of the cylinder to the cap body ( 3b ). as shown in fig2 the thrust lock mechanism ( 6 ) is a mechanism such as used for retracting or extending a pen core such as in a ball point pen or the like . it is composed of a fixing ring ( 9 ) fitted to the upper part of the supporting shaft ( 5 ) and arranged on the supporting shaft below the fixing ring is a rotary ring ( 10 ) in such a way that it may be rotated and slid in its upward or downward direction , engaging projections ( 11 ) and guide grooves ( 12 ) formed in the guide cylinder . the fixing ring ( 9 ) is formed with saw teeth ( 9a ) at its lower edge and fitting claws ( 9b ) spaced apart by 90 ° around a circumferential surface thereof . the lower surfaces of the fitting claws ( 9b ) are similarly inclined as those of the saw teeth ( 9a ). the rotary ring ( 10 ) is formed with saw teeth ( 10a ) at its upper end to be engaged with the saw teeth ( 9a ) of the fixing ring . engaging claws ( 10b ) which are inclined in the same manner as the saw teeth ( 10a ) are formed at the upper surface around the circumferential surface and are spaced 90 ° apart from each other . a spring ( 13 ) is placed between a spring seat ( 5a ) that is fixed substantially at a central part of the supporting shaft ( 5 ) and the rotary ring ( 10 ) and thereby always pushes the saw teeth ( 10a ) of the rotary ring ( 10 ) against the saw teeth ( 9a ) of the fixing ring ( 9 ) under constant pressure . the inner circumferential surface of the guide cylinder ( 3 ) is formed with guide grooves ( 12 ) spaced 90 ° apart from each other into which the fitting claws ( 9b ) of the fixing ring ( 9 ) are fitted between the engaging projections ( 11 ). the lower edge of each of the engaging projections ( 11 ) is formed with inclined sides ( 11a ) for receiving the engaging claws ( 10b ) of the rotary ring ( 10 ). the supporting shaft ( 5 ) is fitted in the guide cylinder ( 3 ) while the fitting claws ( 9b ) of the fixing ring ( 9 ) are fitted in the guide grooves ( 12 ), thereby restricting rotation and is supported in such a way as it may slide in an upward or downward direction . at the same time , it is always biased upwardly by the spring ( 14 ) that is arranged between the spring seat ( 5a ) and the bottom surface of the guide cylinder ( 3 ) in its compressed condition and is stopped at its upper end position where the upper end of the fixing ring ( 9 ) is pressed against the lower surface of the cap body ( 3b ). the plug cap 4 is threaded to the upper end of the supporting shaft ( 5 ). a packing ( 15 ) is fitted to an outer circumference of a lower surface of the plug cap ( 4 ), and the plug is closed when the packing ( 15 ) and the plug cap ( 4 ) are fitted in the concave part ( 1a ) of the drainage plug main body ( 1 ). packing 15 includes a convex - shaped ring portion ( 15a ) that contacts concave part ( 1a ) without sliding relative thereto . the supporting shaft ( 5 ) and the plug cap ( 4 ) are fixed by a threaded part ( 16 ) located at a specified portion of the upper end of the supporting shaft ( 5 ). a fitting part ( 18 ) having a threaded hole ( 17 ) only a its inlet part is arranged at a rear central surface of the plug cap ( 4 ). the threaded part ( 16 ) of the support shaft ( 5 ) is threadably engaged with the threaded hole ( 17 ), and is thereafter fitted to the fitting part ( 18 ). with this arrangement , the supporting shaft can freely rotate with respect to the plug cap as to prevent removal of the plug cap ( 4 ). the packing ( 15 ) is formed in such a manner that its outer circumference depends downwardly so that it can be sufficiently compressed and thereby the plug cap ( 4 ) can be depressed from its closed condition shown in fig3 to the condition shown in fig4 . furthermore , the operation of the thrust lock mechanism and the plug opening and closing operation will be described in reference to fig5 ( a ) to ( d ). fig5 ( a ) to ( d ) illustrate a schematic view in which a surrounding part of the guide grooves ( 12 ) of the guide cylinder ( 3 ) is illustrated from its outer circumference . fig5 ( a ) shows the part of the guide groove ( 12 ) when the plug is opened as shown in fig1 . in this condition , the fitting claw ( 9b ) of the fixing ring ( 9 ) is positioned at an upper end of guide groove ( 12 ), the engaging claw ( 10b ) or the rotary ring ( 10 ) is fitted in the guide groove ( 12 ) and abutted against the lower surface of the fitting claw ( 9b ). under this condition , the saw teeth ( 9a ) of the fixing ring ( 9 ) and the saw teeth ( 10a ) of the rotary ring ( 10 ) are engaged with each other while the end portions of both teeth abut each other . then , as the plug cap ( 4 ) is depressed down to a position shown in fig4 the fitting claw ( 9b ) of the fixing ring ( 9 ) descends down to a lower end of the guide groove ( 12 ) to push out the engaging claw ( 9b ) of the fixing ring ( 9 ) from the lower end of the guide groove ( 12 ). then , the saw teeth ( 10b ) of the rotary ring ( 10 ) slide along the inclined surface of the saw teeth ( 9b ) of the fixing ring ( 9 ) in a leftward direction with the biasing force of the spring ( 13 ) and thus both saw teeth ( 9b ) and ( 10b ) completely engage each other as shown at fig5 ( b ). when a hand is released from the plug cap ( 4 ) that is depressed down to a condition shown in fig4 the supporting shaft ( 5 ) lifts under the force of the spring ( 14 ), and the rotary ring ( 10 ) is pushed up by the spring seat ( 5a ), thereby as shown in fig5 ( c ), the engaging claw ( 10b ) of the rotary ring ( 10 ) is engaged with the inclined side ( 11a ) at the lower edge of the engaging projection ( 11 ) of the guide cylinder ( 3 ). therefore , the engaging claw ( 10b ) of the rotary ring ( 10 ) is engaged with the inclined side ( 11a ) at the lower edge of the engaging projection ( 11 ), thereby locking the supporting shaft ( 5 ) at its lower end and the plug cap ( 4 ) is also locked in its closed condition as shown in fig3 . then , if the plug cap ( 4 ) under its closed condition is depressed down to a position shown in fig4 saw teeth ( 9a ) of the fixing ring ( 9 ) depress the end part of the saw teeth ( 10a ) to cause the engaging claw ( 10b ) of the rotary ring ( 10 ) engaged with the inclined side ( 11a ) to be disengaged downwardly . then , the saw teeth ( 10a ) of the rotary ring ( 10 ) slide in a leftward direction along the inclined side of the saw teeth ( 9a ) of the fixing ring ( 9 ) under a biasing force of the spring ( 13 ). then both saw teeth ( 9a ) and ( 10a ) are completely disengaged from each other as shown in fig5 ( d ). then if the plug cap ( 4 ) is released , the fitting claw ( 9b ) of the fixing ring ( 9 ) may ascent within the guide groove ( 12 ). simultaneously , the supporting shaft ( 5 ) and the rotary ring ( 10 ) are lifted up with the force of the spring ( 14 ) and after the engaging claw ( 10b ) of the rotary ring ( 10 ) abuts against a lower end port of the guide groove ( 12 ), it may ascend within the guide groove ( 12 ) together with the fitting claw ( 9b ) and returned back to the condition shown in fig5 ( a ). as described above , since the drainage plug ( a ) can be opened or closed under a manual depression of a user of the plug cap ( 4 ) from above , the opening or closing operation can easily be performed . although the preferred embodiments are illustrated for a case in which the present invention is applied to a basin , the present invention can also be applied in a bathtub and in this case the plug cap is depressed down by a foot depressing action of the user . fig6 and 7 illustrate another preferred embodiment of the present invention in which an operating lever ( 20 ) is connected to the plug cap ( 4 ). in this preferred embodiment , the lower end of the operating lever ( 2 ) is connected to the plug cap ( 4 ) by a screw or the like or the plug cap ( 4 ) and the operating lever ( 20 ) can be integrally formed . a pressing surface ( 22 ) is formed at the upper end of the operating lever ( 20 ). the operating lever ( 20 ) is rigid and is made by a material having a small as possible deformation and strain caused by its depressing action . for example , the material may be either a hard resin or a light metallic material . the pushing surface ( 22 ) projects upwardly from the surface ( 24 ) of the accumulated water in the reservoir ( b ). in order to perform this operation , the pushing surface ( 22 ) may be arranged either at the same level or at a higher level than that of the overflow port ( 26 ) of the reservoir ( b ). the operating lever ( 20 ) is preferably formed as an upwardly curved shape along an inner circumferential surface near the drainage port ( 2 ) of the reservoir ( b ), thereby preventing the operating lever ( 20 ) from being disturbed during use of water in the reservoir ( b ). in this way , in case that the accumulated water in the reservoir ( b ) is to be discharged , the pushing surface ( 22 ) of the operating lever ( 20 ) is manually pushed down causing the plug cap ( 4 ) to descend , enabling the abovementioned plug opening operation to be performed . therefore , the cap can be opened without inserting the hand of the operator into the reservoir and the water can be discharged .