PURPOSE: To prevent a gate from cracking and to measure pressure resistance without center deviation by pressing a post against the cathode part of an element and providing a guide which has through holes at corresponding places above the cathode.

CONSTITUTION: A support base 11 made of an electrically conductive material is mounted on a table 8 and the element 12 is held in the annular groove 14 of a fitting part 13. The post 16 is arranged on the element 12 and contacts the cathode of the element 12 electrically. The cylindrical guide 17 is arranged outside the post 16, the lower end part of the guide 17 is fitted to the upper part 11a of the support base 11, and the post 16 penetrates its upper end part. The guide 17 has four through holes 17a at corresponding positions on the gates of the element 12 and contact measuring elements 18 which contact the gates electrically are inserted respectively and connected to connection terminals 19.