A method of producing a polymer insulator having a core, an overcoat member arranged on the core, and a fitting member for securing the core at its both ends, wherein the core and the fitting member are secured under pressure at both ends of the core by contacting a ring-shaped protrusion portion arranged at both ends of the overcoat member to an open inner end of the fitting member, includes the steps of: forming the overcoat member on the core; forming the ring-shaped protrusion portion integrally with the overcoat member by processing an end portion of the overcoat member; and securing the fitting member. Moreover, an end processing apparatus utilized for the method of producing a polymer insulator includes: a core holder for holding a core; a rotation drive device arranged rotatably around the core holder at a center of a central axis of the core; and a whetstone having a shape for forming an end portion of an overcoat member of the polymer insulator, which is arranged to the rotation drive device rotatably around its center axis; wherein the whetstone itself is rotated and the rotated whetstone is rotated around the end portion of the overcoat member on the core held by the core holder, so that a protrusion portion is formed integrally with the end portion.