An insulator includes a housing portion. The housing portion is composed a silicone rubber obtained by curing a composition containing diorganopolysiloxane (A) and a fine silicon powder (B) by heating, and a methylalkylpolysiloxane oil (C). The diorganopolysiloxane has at least two silicon atom-bonded alkenyl groups in one molecule. The methylalkyl-polysiloxane oil (C) has each of both terminal ends of each molecular chain blocked with a trialkysiloxy group and possessing a viscosity at 25 DEG C. being 1 to 1,000 centistokes, and is incorporated into the above composition in an amount of 1-100 parts by weight per 100 parts by weight of the component (A).
