PURPOSE: To measure the degree of vacuum in a cathode-ray tube, by a method wherein an emission flows between a cathode of an electron gun of a cathode- ray tube and an arbitrary electrode, and ion produced as a result of said flow is gathered at other electrode.

CONSTITUTION: A cathode K, a first grid G1, a second grid G2, and a fourth grid G4 are located in an electron gun of a cathode-ray tube. An emission flows between the cathode K and other electrode. Ion produced as a result of the collision of the emission with a gas molecule is collected to the second grid G2 having a negative voltage. An ion current I2 has a correlative relation with the degree of vacuum and this allows to measure the degree of vacuum in the cathode- ray tube.