PURPOSE: To accurately measure the energy absorbed by a resist even if the spectrum of applied X ray changes.

CONSTITUTION: An electrode 4 where a voltage is applied is laid out inside a closed ionization box and a gas containing an element composition (excluding hydrogen and helium) which is virtually equal to the resist exposed to X ray is filled. When X ray 1 enter from an incidence window 3, secondary electrons corresponding to X-ray intensity are generated and the X-ray intensity can be measured by measuring the ionization current of ionized gas. Since the wavelength dependency of detection sensitivity is equal to that of resist, use of the detector output for resist exposure improves the accuracy for controlling amount of exposure.