PURPOSE: To easily control an exposed radiation quantity, by constituting a device so that a worker can automatically operate probability exceeding a prescribed exposed radiation quantity.

CONSTITUTION: An area which contains a range where a worker moves, and also is influenced by a radiant ray source extending over the whole area is set as a unit work area 2, and on this unit work area 2, radiant ray detectors 4aW4c are provided. An operating circuit 5 derives a pattern of a dose rate distribution of the unit work area 2, and cumulative percentage of the does rate in accordance with the does rate distribution in the unit work area 2, which has been inputted in advance, and the worker calculates probability exceeding a prescribed exposed radiation quantity, and also performs a correction in accordance with a signal from the radiant ray detectors 4aW4c. A result of operation of the operating circuit 5 is displayed on an indicator 7, and also is printed out by a printer 8.