In order to achieve high sensitivity without an increase in device complexity or cost, a far-ultraviolet absorbance detection device for liquid chromatography is provided with: an optical system including a light source that emits light including far-ultraviolet light, a diffraction grating for dispersing the light emitted from the light source, a flow cell through which a liquid is passed, a slit for selecting a predetermined wavelength of +1 order light diffracted by the diffraction grating and causing the light to enter the flow cell, a first photodetector for detecting the light transmitted by the flow cell, and a second photodetector for detecting light other than the +1 order light diffracted by the diffraction grating; a mechanism for evacuating or substituting the optical system with nitrogen gas; and a computation unit that calculates absorbance from an output signal from the first photodetector and an output signal from the second photodetector. The second photodetector is fixedly disposed.