To provide novel devices constituting light detection systems with a simple configuration, a low maintenance cost, a reduced size and weight, and high reliability by integrating light detection components and power supplies.A photosensitive solid-state thin-film lithium-ion secondary cell including a photosensitive negative-electrode active material layer as a component, wherein extraneous light can be constantly detected by utilizing a change in open-circuit voltage of the cell caused when the extraneous light is incident on the photosensitive negative-electrode active material layer. Lithium ions can be readily received and emitted and light in a broad wavelength range over a near-infrared light range, the entire visible light range, and a near-ultraviolet light range can be detected by utilizing a porous silicon layer having a controlled porosity as the negative-electrode active material layer.