A lightning arrestor insulator is provided having a discharge gap portion and an arrestor ZnO element device (5) both in the body of the insulator, comprising opposed projecting discharge electrodes (3a,3b) arranged inside the insulator body. The discharge gap portion is formed of a heat resistant protrusion (2) inside the insulator body surrounding the discharge electrodes (3a,3b), and a pair of metal plates and/or electrically conductive ceramic plates (4a,4b) sandwiching the protrusion (2) from both sides thereof and electrically connected to the discharge electrodes. The pair of plates (4a,4b) are joined and airtightly sealed to the protrusion via an inorganic glass (10a,10b). The arrestor has a highly reliable airtight fixing and sealing structure so that accidental troubles in a power supply or distribution line at a normal working voltage can be substantially eliminated, and damage caused by hygromeration and lightning can be noticeably decreased.