An optical concentration measuring device includes a light source, detectors (21, 22) that detect a signal, based on light emitted from the light source, and output a detection signal, a signal processor (50) that acquires the detection signal and converts the detection signal to a digital signal, and a substrate (30) on which the detector and the signal processor are provided. The signal processor has a terminal array in which a plurality of signal terminals is sandwiched between a plurality of voltage terminals, the signal terminals being input and output terminals of signal lines electrically connected to the detectors, 10 the voltage terminals supplying a voltage other than 0 V. In plan view of the substrate from the front, the signal processor and wiring connecting the plurality of voltage terminals surround the signal lines.