PURPOSE: To perform monitoring of degree of vacuum by proving a Kerr element with a polariface changeable according to an electric field in the neighborhood of a vacuum part of an electric equipment the electric field thereof varies according to the degree of vacuum to detect the light from a light source through a polarizer by an analyser.

CONSTITUTION: A part having a large change of electric field according to the quanlity of the degree of vacuum inside of an electric appliance having a vacuum part, for instance, a vacuum breaker consisting of a fixed electrode 1, movable electrode 2, fixed lead 3, movable lead 4, insulating tube 5, end plates 6 and 7 and a shield 9 is provided with a Kerr element 21 changing a polariface according the strength of an electric field. The Kerr element is irradiated from a light source 23 through an optical fiber 24, polarizor 20 to obtain the output of the analyser in the electric signals through the optical fiber 26 at a photoelectricity conversion part 25 to judge the degree of vacuum at a vacuum grade judgment part 27. When the degree of vacuum is satisfactory, 1/2 of the terminal voltage V is impressed on the shield 9, while when the degree of vacuum is deteriorated, the voltage rises to be come 3/4 of instance. Thereby, the polariface of the Kerr element changes and the quantity of transmitting light changes thus enabling to detect the deteriorated degree of vacuum.