An electronic cassette and a radiation imaging system capable of preventing information leakage from a universal memory device and improving usability. A USB memory device 30 incorporates a flash memory 32 and a memory controller 33. An access lock is placed on a data storage area 32b of the flash memory 32. When the USB memory device 30 is attached to an electronic cassette 11, a cassette controller 55 controls a USB controller 60 to input an unlock password to the USB memory device 30. The memory controller 33 unlocks the access lock on the data storage area 32b when the unlock password matches a password stored in a program area 32a in advance.
