First and second pulse height detection circuits output pulse height detection signals which rise when a detection pulse obtained from a radiation detector becomes greater than a lower threshold Lsh or an upper threshold Hsh, and fall when the detection pulse is smaller than the lower threshold Lsh or the upper threshold Hsh. Next, first and second rising and falling detection circuits detect rising and falling edges of the pulse height detection signals from the first and second pulse height detection circuits in synchronization with a clock pulse from a crystal oscillator, and a combining circuit outputs a signal corresponding to the detection pulse that is within a range between the lower threshold Lsh and the upper threshold Hsh by combining both outputs from the first and second rising and falling detection circuits, in synchronization with the clock pulse.