According to one embodiment, an automatic analyzer comprises a light source, a spectroscope, a photo detection unit, a storage unit, a selection unit, and a calculation unit. The storage unit stores photo detector identifiers related to photo detectors and wavelength band identifiers in association with each other. The selection unit selects a specific photo detector from photo detectors. The specific photo detector corresponds to a specific photo detector identifier associated with a wavelength band identifier of a wavelength band according to a measurement item of a sample. The calculation unit calculates an absorbance related to the measurement item based on a signal from the selected specific photo detector.