This telephone system is composed of telephones and safety maintaining unit. This telephone set located in explosive hazardous area can be connected to the telephone lines of marine telephone exchange, PABX (Private Automatic Branch Exchange) or analog telecom network via Buffer Unit. With its safe security system, the equipment can be operated for a long period of time in a dangerous environment such as oil carrier ships, LNG/LPG carrier ships, chemical carrier ships. Specified to the intrinsically safe explosion proof construction grade that satisfies the ia standard, assuming two accidents, in compliance with the IEC specification, meets the requirement of the Ex ia IIBT4 class of equipment in terms of explosion proof and inflammable nature. The authentication of the IECEx has acquired. Ex. Protection : ODA-1371-1A & 1AH (Ex ia II BT4 Ga Ta= -20℃～+55℃), ODZ-9440-1A ([Ex ia] II B Ta= -20℃～+60℃)