Design and parameters of the photoionization quantometer are given. It is a combination of photoionization chamber with time‐of‐flight mass spectrometer. The quantometer is an absolute detector of radiation in the wavelength interval (100–0.1 nm) due to the fact that photoionization and electron‐impact ionization cross sections of gases are used for absolute measurements. The quantometer is suitable for measurements of the number of photons in radiation pulses. The minimum number of photons detectable for a single pulse increases from 106cm−2puls−1at 100 nm up to 1011cm−2puls−1for 0.1 nm. The quantometer has been used for vacuum ultraviolet and soft x‐ray radiation measurements in the laser plasma experiment. An improved variant of the quantometer is presented which provides the absolute measurements of soft x‐ray radiation for laser plasma having continuous spectrum. The inaccuracy of measurements does not exceed 25% for soft x ray.