PURPOSE: To provide a γ-ray level detector capable of performing stable and highly accurate level detection by using a physically stable reference signal in a γ-ray level detector using a bar-shaped scintillator.

CONSTITUTION: A reference ray source 5 as the ray source of a capacity other than a legally regulated capacity is fixed in the vicinity of a photomultiplier 6 opposite the measuring face of a measuring device 4 and used as a standard reference light. A signal outputted from the phtomultiplier 6 is supplied, via an amplifier 7, to a measuring signal part 8 for calculating a measuring rate based on a low discrimination level DS-1 and a measuring signal part 9 for calculating a measuring rate based on a high discrimination level DS-2. Signals outputted from the measuring signal parts 8 and 9 are subtracted from each other by a subtractor 10A. The result of this subtraction is compared with a specified value by a comparator 10B and a control signal is supplied to a high pressure part 11 so that the subtraction result is equal to the specified value. A signal outputted from the measuring signal part 8 is supplied to a level calculating part 27 and the level of the content of vessel 2 is calculated.