PURPOSE: To freely set a pressure at a boundary in which an emission current is to be increased or decreased by a method wherein a circuit which compares a pressure value with a set value is connected to an emission control circuit, a circuit which outputs the set value is made variable and its output is changed via an output circuit.

CONSTITUTION: When an ionization gauge is actuated, a filament is lit up, a prescribed minimum emission current flows and a pressure measurement is started. Then, a signal corresponding to a pressure is output from a pressure- signal output circuit 1, its output is compared with a high-pressure-side set value HP at a setting circuit 5 by a comparison circuit 2, and it is compared with a low-pressure-side set value LP by a comparison circuit 3. According to outputs from the comparison circuits 2, 3, an emission control circuit 4 is actuated, and an emission current corresponding to the pressure is changed over. When an output from the setting circuit 5 is changed via an input circuit 6, the HP and/or LP values as a boundary in which the emission current is to be incraesed or decreased can be changed.