For the purpose of applying the laser-ionization method to measuring gas pressure, particularly in the range of UHV and XHV, the laser-ionization characteristics for various kinds of gases have been studied using XeCl, KrF and ArF excimer lasers. The proportional relation between the ionization signal and the gas pressure was experimentally verified to 1 × 10^-8 Torr (1.3 × 10^-6 Pa). In the case of rare gases and O₂ gas, the dependence of the ionization signal on the laser intensity was in agreement with the theory of nonresonant multiphoton ionization. Saturation of ionization was observed in the case of Xe.