To provide an ultrahigh vacuum gage capable of adjusting and measuring offset derived from a gage head and the whole measuring system, and an ultrahigh vacuum measuring method using the gage.

In this ultrahigh vacuum gage, a condition wherein ions do not pass an energy analyzer is set, and a circuit means for measuring and adjusting the offset of the whole measuring system such as a residual current by soft X-rays, an amplifier, a cable and a connector part is provided. This ultrahigh vacuum measuring method is constituted as follows: in the vacuum gage for ionizing gas molecules by electron impact at an ion source constituted from an electron source and a grid working simultaneously as an ion generation part, and measuring the pressure by detecting generated ions by an ion detection part through the energy analyzer, an offset measuring and adjusting circuit means is combined with the energy analyzer and used, and the energy analyzer is set at an offset adjusting mode condition wherein the ions do not pass the energy analyzer, and under this condition, the offset of the whole entire measuring system such as the residual current by the soft X-rays, the amplifier, the cable and the connector part is measured and adjusted.