To apply a prescribed voltage in a short time in an emergency since time is taken until bias voltage reaches a rated value and the bias voltage must be applied on an FPD at a prescribed voltage increase rate.

This radioactive ray detector has a bias power source 9 for controlling the bias voltage applied on a converting membrane 23 for converting a radioactive ray into electric charge so as to gradually reach the rated value on the basis of the prescribed voltage increase rate, and has a voltage holding part 201 so as to restrain a sudden drop in the bias voltage applied on the converting membrane 23 when cutting of power source supply to the bias power source 9. Thus, when the power source supply is restored, voltage can be quickly increased to the prescribed high voltage from a holding voltage.