To accurately and stably measure a dynamic range that is wide from a low radiation level to a high radiation level.

An ionization-box-type radiation detector for detecting radiation by measuring an ionization current due to the incidence of radiation by applying the measuring principle of an ionization box is provided with first and second cathode electrodes 11 and 12 where each different voltage is applied and an anode electrode 13. The radiation detector is provided with a pulse count means 19 by alternately arranging a plurality of at least one second cathode electrode 12 and anode electrode 13 at a position opposite to the first cathode electrode 12, inputting a signal from the anode electrode 13, and outputting a pulse each time when a signal with a larger signal than a predetermined wave-height discrimination level is inputted and an external output means 110 that inputs a pulse from the pulse count means 19 for performing operation, instructing and outputting the operation result, or has a function for outputting a signal to the outside in a signal form being requested from the outside such as an alarm signal.