Method for inspecting insulator for spark plug

A method for inspecting an insulator for a spark plug to determine whether or not the insulator has a defect, the spark plug extending in an axial direction, the method comprising:

• a voltage applying step of applying a voltage between a first electrode disposed in an axial hole in the insulator and a second electrode disposed near an outer peripheral surface of the insulator,
• wherein the voltage applying step is performed while a gap between a front end of the first electrode and an inner peripheral surface of the insulator that faces the front end of the first electrode is filled with a first insulating material without leaving a hollow space;
• and
• wherein, in the voltage applying step, a front distal tip of the first electrode is located in the axial hole.