PURPOSE: To enable highly accurate detection of abnormal conduction based only on pressure, even in case of an outdoor facility, by an arrangement wherein 4 pressure difference and level judging means makes a judgment based on information provided from a pressure information forming means.

CONSTITUTION: Gas pressure information and an abnormality judgment value are obtained from an input amplifier 4, a multiplexer 5, an A/D converter 6 and a data memory 6 at a monitoring section 3, i.e., a pressure information forming means. A pressure difference judging means, i.e., a pressure rise judging/ operating section 8, and a pressure level judging means, i.e., an absolute value judging/operating section 9, then judges existence of abnormal conduction based on a differential gas pressure and a gas pressure level while eliminating the influence of outdoor air. Abnormality is notified by one of the judging means 8, 9. Consequently, abnormal conduction can be detected and monitored positively with high accuracy through a simple arrangement for measuring only gas pressure even when the gas pressure increases gradually due to local abnormality in an outdoor electric facility.