PURPOSE: To obtain a method (device) which can discriminate the reliability level of the measured results of the gas ratio of a cathode ray tube by a gas ratio measuring method (measuring instrument).

CONSTITUTION: The gas ratio of a cathode ray tube 1 is measured by adjusting the first negative voltage applied across the first grid G1 of the tube 1 from a power source EC1 so that a prescribed reference current can be made to flow to the cathode K of the tube 1 when the second negative voltage of a negative power source E1 is applied across the terminal pins P of the tube 1 by switching a change-over switch SW and measuring the current which flows to the anode terminal Ta of the tube 1 when the second negative voltage of the power source Ea is applied with an ammeter AM. Then the contact resistance between the anode terminal Ta and a connector is measured by measuring the current flowing to the terminal Ta when a reference potential is applied across the terminal pins P by switching the switch SW and the measured result of the gas ratio of the tube 1 is evaluated based on the measured result of the contact resistance.