An object of the present invention is to provide a vacuum tube with a structure close to that of an inexpensive and easily available vacuum fluorescent display which easily operates as an analog amplifier. A vacuum tube subject to the present invention comprises: a filament which is tensioned linearly and emits thermoelectrons, an anode arranged parallel to the filament, and a grid arranged between the filament and the anode such that the grid faces the anode. The present invention is characterized in that a distance between the filament and the grid is between 0.2 mm and 0.6 mm, including 0.2 mm and 0.6 mm.