PURPOSE: To form a micro vacuum tube in the form of a planar by processing the forward end of a cathode of a width of a very thin wire to be thinner by a focused ion beam FIB, and applying an isotropic etching to an insulation substrate or an insulation layer under each electrode after forming an anode and a grid, to let the forward end float.

CONSTITUTION: A cathode electrode 3 is formed in a width of less than a submicron on an insulation layer 2 on a substrate 1 by combination of a pictured image of a FIB and a deposition liftoff, and its forward end is processed to be thinner by the FIB to facilitate emission of electrons. An anode 4 and a grid electrode 5 are disposed and formed as predetermined, an isotropic etching is applied to the insulation layer 2 under each electrode, so the forward end of each electrode floats up from the substrate 1. In this constitution, the form of the cathode 3 and the distance between the cathode and anode can be uniform in composing a planar type micro vacuum tube, it can be formed on an existing planar type semiconductor IC simultaneously with other elements, and by laminating a material of a small work function on a high fusing point metal of the cathode 31, emission of electrons is facilitated further. A linear anode facing a knife-like cathode with a grid electrode provided at an interval is also effective.