A screw measuring apparatus comprising: an internal-dimension measuring unit for obtaining internal-dimension information Dw of a female screw placed on a coordinate measuring machine when the stylus-tip sphere having the diameter selected according to the pitch of the female screw is butted against the screw groove of the female screw; and an effective-diameter calculating unit for obtaining effective-diameter information De of the female screw from the internal-dimension information Dw obtained by the internal-dimension measuring unit, diameter information 2 r of the stylus-tip sphere, and thread angle information α and pitch information P of the female screw, by using an effective-diameter calculation expression De=Dw+2 r×cos ec(α/2)−(P/2)×cot(α/2).