© 2019 Elsevier B.V. Recently, a focus has been placed on plasmonics utilizing ultraviolet light because of its higher energy and shorter wavelength, compared to visible light. Al is a suitable metal for plasmonics in the ultraviolet region. However, Al is easily oxidized, and accurate control for Al film deposition is difficult. In this study, organic thin films were formed on the Al films, and their surface plasmon resonance (SPR) wavelengths were measured by our original attenuated total reflectance spectroscopic system. With an increase in the organic film thickness, the SPR wavelength shifted to longer wavelengths. The minimum detection thickness of the organic overlayer reached 2 nm for three Al films under different conditions (i.e., different Al thickness and oxidation effects), and the spectra were reproduced by simulations based on the Fresnel equations. These results showed the practical advantage of the Al-based SPR sensor without accurate control of the chemical state of the Al film.