PURPOSE: To improve the control precision of accelerating voltage by providing a voltage compensating circuit between the middle point of a voltage dividing circuit and the transistor base of an active load circuit.

CONSTITUTION: Thermoelectrons are emitted from the hair pin section (b) of a filament F to an anode AN, a current flows to an accelerating voltage source E1, and the potential at the connection point (c) of transistors Q1, Q2 of an active load circuit ALC is fluctuated and made inconsistent with the potential at the middle point (a) of a voltage dividing circuit RC. When the potential at the point (c) is made lower than the potential at the point (a), a control signal for compensating the voltage to a high level is outputted from a differential amplifier Amp. The current fed back to the power source E1 is increased or decreased to make the potential at the point (c) equal to the potential at the point (a) of the circuit RC. The potential at the point (c) of the circuit ALC is invariably made equal to the potential at the point (a), the potential at the hair pin section (b) is also made equal. The accelerating voltage set by the power source E1 is made equal to the accelerating voltage applied across the filament F and the anode AN, and the accelerating voltage can efficiently be controlled.