A signal processing circuit (123) includes a detection unit (126) configured to detect generation of a peak in an analog signal based on an input of a photon, an A/D conversion unit (202) configured to perform A/D conversion of a signal value into digital data of bits by determining a value of each of the bits from an upper bit to a lower bit, and a control unit (201) configured to control the A/D conversion unit so that, in a case in which the generation of a second peak of the analog signal is detected during the A/D conversion of a signal value of a first peak of the analog signal, the A/D conversion of the signal value of the first peak will be interrupted and the A/D conversion of a signal value of the second peak will be started.
