Light emitted in correspondence to ionizing radiation incident from a scintillator is fed through an optical fiber to a photoelectron multiplier tube by which it is converted to an electrical signal. The electrical signal is amplified by a signal amplifying circuit, and any light emission events of given or higher intensity are discriminated by a discriminator and counted by a counter. The count value is fed to a computer. The computer converts the count value to a dosage on the basis of an exponential relationship lying between the light emission intensity and the number of emission events, thereby attaining detection of the dosage.