The invention discloses a signal processing device and a signal processing method. According to an embodiment, a signal processing device includes an integrator, a first analog-to-digital converter, and a histogram creator. The integrator is configured to integrate an electrical charge corresponding to electromagnetic waves. The first analog-to-digital converter is configured to perform an analog-to-digital conversion operation that generates digital data of the electrical charge using an integration output from the integrator, on a parallel with an integration operation performed by the integrator. The histogram creator is configured to create a histogram that represents an energy distribution of the electromagnetic waves, from the digital data generated by the first analog-to-digital converter.