Provided is an organic thin-film solar cell, including: a substrate, an anode, an organic thin-film layer that includes an organic semiconductor layer, and a cathode. The anode, the organic thin-film layer that includes the organic semiconductor layer, and the cathode are layered in order on top of the substrate. A recess and protrusion-shaped microstructure that includes a plurality of recesses or protrusions arranged two-dimensionally at random is formed in an interface between the organic thin-film layer and the cathode. The recess and protrusion-shaped microstructure is formed such that, when λ1 and λ2 are a shorter wavelength and a longer wavelength, respectively, of wavelengths that produce an absorption edge in a light absorption spectrum of the organic semiconductor layer, and k1 and k2 are real parts of propagation constants of surface plasmons that correspond, respectively, to those wavelengths and occur along an interface between the organic semiconductor layer and the cathode, and when the real part k1 corresponds to an upper wavenumber limit K1 in a power spectrum of a height distribution of the microstructure formed in the interface between the cathode and the organic semiconductor layer, and the real part k2 corresponds to a lower wavenumber limit K2 in the power spectrum of the height distribution of the microstructure formed in the interface between the cathode and the organic semiconductor layer, the power spectrum of the height distribution of the microstructure exhibits determinate values between the upper wavenumber limit K1 and the lower wavenumber limit K2, and an integrated value of a spectral intensity of the power spectrum of the height distribution over a wavenumber range from K1 to K2 is equal to at least 50% of an integrated value of the spectral intensity of the power spectrum of the height distribution across all wavenumbers.