the above internal screw ridge 53 sizes of the inclination angles of its flanks are made inverse to those of the inclination angles of the flanks of a standard inverse buttress external screw this screw 51 corresponds to an inverse buttress screw in a state that the position of a pitch diameter hereinafter referred to as the standard pitch diameter 6 of such a standard inverse buttress screw as shown in fig5 is shifted to a predetermined position namely a position of a pitch diameter 58 of the internal screw 52 of the inverse buttress screw 51 is such a position that the width p 3 of the internal thread ridge 53 in the position of the standard pitch diameter 6 is made greater at a predetermined ratio than the width p 4 of a thread groove 59 this predetermined ratio p 5 p 6 is similar to the predetermined rate p 3 p 4 of the first embodiment the present invention is not limited to the above embodiments for example the present invention can be applied to a screw other than the triangular screw and the inverse buttress screw and also it can be applied not only to the internal screw but also to the external screw by using the above first and second embodiments and an accumulator made by nippon accumulator co ltd type n 210 1d of a prior art example the inventor analyzed a tensile stress value in a first thread valley bottom of the internal screw by a finite element method the following results were obtained incidentally the u201c first thread valley bottom u201d means a first thread valley bottom of a base portion of the internal screw which initially undergoes a load of the external screw metric internal thread of the prior art example.