PURPOSE: To attain to enhance measuring capacity by reducing a measuring error, by calculating pressure from the radio of the charging time of the ion current due to an integrator and the discharge time of the integrated amount due to electron current control.

CONSTITUTION: An integrator 13 is charged with the ion current of the ion collecting electrode 4 in an ionization type pressure measuring sphere 1 for a predetermined time and, succeedingly, the charge charged in the integrator 13 is discharged by the current signal of an electron collecting electrodes 5. In this case, the time before discharge potential reaches zero from the start of discharging is measured by the output control of a comparison circuit 14 for detecting said discharging in a counter 15 and the pressure in the measuring sphere 1 is calculated from the ratio of the charging time and the discharging time. As a result, the measuring error caused by the variation of the electron current or the capacity change of an integration capacitor can be removed and can be directly read by digital display without providing a special circuit and measuring capacity can be enhanced.