PURPOSE: To relatively easily grasp the quantity of change with the lapse of time of the mutual space between a cathode and a grid electrode of an electron gun and the cut-off voltage at a low cost.

CONSTITUTION: A double-bulb, which is provided with a cathode 12 manufacture with the design reference as same as the cathode to be incorporated in an electron gun for CRT and an anode 14 arranged in parallel with an emitter 13 of the cathode 12, is manufactured. This double-bulb is operated in a space charge limited area for a long time, and the voltage-current characteristic thereof at the initial time and the later time is measured so as to compute the mutual space between the cathode 12 and the anode 14 of this double-bulb on the basis of the measured values and a relational formula of the voltage-current of the double-bulb. Quantity of change with the lapse of time of the mutual space between the cathode and the grid electrode of the electron gun is presumed on the basis of the quantity of change with the lapse of time of the computed mutual space. The cut-off voltage is obtained by the electric field simulation.