A screw measuring method comprising: an internal-dimension measuring step of using a coordinate measuring machine provided with a screw measuring probe that has a stylus-tip sphere having a diameter selected according to the pitch of a female screw to measure internal-dimension information Dw equal to the diameter of a cylinder having a center axial line identical with the axial line of the female screw and having a wall that passes through the center of the stylus-tip sphere when the stylus-tip sphere is butted against the screw groove of the female screw, based on coordinate information measured while the stylus-tip sphere is butted against the screw groove of the female screw; and an effective-diameter calculating step of obtaining effective-diameter information De of the female screw from the internal-dimension information Dw measured in the internal-dimension measuring step, a diameter 2r of the stylus-tip sphere, and a thread angle α and a pitch P of the female screw, by using an effective-diameter calculation expression De = Dw + 2r x cosec(α/2)-(P/2) x cot(α/2), wherein the internal-dimension measuring step comprises: a screw-groove detecting step of detecting the screw groove at two or more measurement points separated by a predetermined angle circumferentially on the wall surface of the female screw, when the stylus-tip sphere is at a maximum displacement point toward the outside in the radial direction of the female screw while the stylus-tip sphere contacts the wall surface of the female screw and the stylus-tip sphere is made to crawl along the axial line of the female screw, the maximum displacement point being regarded as a point where the stylus-tip sphere completely falls in the screw groove of the female screw; and an internal-dimension calculating step of obtaining the internal-dimension information Dw according to coordinate information on each screw groove detected in the screw-groove detecting step, wherein the cylinder has the wall that passes through the maximum displacement points. an internal-dimension measuring step of using a coordinate measuring machine provided with a screw measuring probe that has a stylus-tip sphere having a diameter selected according to the pitch of a female screw to measure internal-dimension information Dw equal to the diameter of a cylinder having a center axial line identical with the axial line of the female screw and having a wall that passes through the center of the stylus-tip sphere when the stylus-tip sphere is butted against the screw groove of the female screw, based on coordinate information measured while the stylus-tip sphere is butted against the screw groove of the female screw; and an effective-diameter calculating step of obtaining effective-diameter information De of the female screw from the internal-dimension information Dw measured in the internal-dimension measuring step, a diameter 2r of the stylus-tip sphere, and a thread angle α and a pitch P of the female screw, by using an effective-diameter calculation expression De = Dw + 2r x cosec(α/2)-(P/2) x cot(α/2), wherein the internal-dimension measuring step comprises: a screw-groove detecting step of detecting the screw groove at two or more measurement points separated by a predetermined angle circumferentially on the wall surface of the female screw, when the stylus-tip sphere is at a maximum displacement point toward the outside in the radial direction of the female screw while the stylus-tip sphere contacts the wall surface of the female screw and the stylus-tip sphere is made to crawl along the axial line of the female screw, the maximum displacement point being regarded as a point where the stylus-tip sphere completely falls in the screw groove of the female screw; and an internal-dimension calculating step of obtaining the internal-dimension information Dw according to coordinate information on each screw groove detected in the screw-groove detecting step, wherein the cylinder has the wall that passes through the maximum displacement points. a screw-groove detecting step of detecting the screw groove at two or more measurement points separated by a predetermined angle circumferentially on the wall surface of the female screw, when the stylus-tip sphere is at a maximum displacement point toward the outside in the radial direction of the female screw while the stylus-tip sphere contacts the wall surface of the female screw and the stylus-tip sphere is made to crawl along the axial line of the female screw, the maximum displacement point being regarded as a point where the stylus-tip sphere completely falls in the screw groove of the female screw; and an internal-dimension calculating step of obtaining the internal-dimension information Dw according to coordinate information on each screw groove detected in the screw-groove detecting step, wherein the cylinder has the wall that passes through the maximum displacement points.