In a signal processing device of an embodiment, an integration circuit accumulates a charge from a photodiode in an integrating capacitor element, and outputs a voltage value according to the amount of charge. A comparator circuit, when the voltage value from the integration circuit has reached a reference value, outputs a saturation signal. A charge injection circuit, in response to the saturation signal, injects an opposite polarity of charge into the integrating capacitor element. A counter circuit performs counting based on the saturation signal. A holding circuit holds the voltage value from the integration circuit. An amplifier circuit outputs a voltage value that is K times (where K>1) larger than the voltage value held by the holding circuit. An A/D converter circuit sets a voltage value that is K times larger than the reference value as the maximum input voltage value, that is, a full-scale value, and outputs a digital value corresponding to the voltage value from the amplifier circuit.