In a signal processing device, a branch section generates, from an input signal which is a current signal, a plurality of branch signals that are proportional to the input signal and have different signal intensities, and supplies the plurality of branch signals to respective different individual paths. A selection section selects one of the plurality of individual paths and outputs a signal supplied through the selected individual path. A determination section determines whether in each of the plurality of individual paths, a magnitude of a signal supplied to the selection section is in a preset allowable range. A control section causes the selection section to select the individual path having a highest gain among the individual paths in which the magnitude of the signal is determined by the determination section to be in the allowable range.