PURPOSE: To simplify a device by connecting in series at least two sets and more of Kerr elements and polariging plates minitoring a degree of vacuum of a vacuum breaker in one closed circuit.

CONSTITUTION: In the case of making a high-class voltage circuit wherein a plurality of vacuum breakers are arranged in a series, for instance, in case two vacuum breakers are arranged in a series, between the main termonals 1 and 1 the breakers 2' and 2" consisting of a fixed electrode 3, movable electrode 4 and an insulating porcelain tube 5 are provided, Kerr elements are arranged for the electric field detection parts D1 and D2 having a large change of electric field owing to the deteriorated degree of vacuum of each breaker to detect a quantity of light at a light receiving part 12 by connecting the polarizers 13AW13C, Kerr elements 14A and 14B inseries as shown in the Figure from a light source 8 through an optical filber. The output thereof is given to a judging part 17 of the degree of vacuum for judging the quality of vacuum. The polarizer, Kerr element and fiber are consisted of an insulating material thus enabling the connection supervising three phases by one closed circuit except a serial connection for one phase. Thereby correct judgment of the degree of vacuum can be performed with a simple device requiring no change in the construction of the breaker.