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Surface incident solar radiation (Rs) plays an essential role in climate change on Earth. Rs can be directly measured, and it shows substantial variability, i.e., global dimming and brightening, on decadal scales. Rs can also be derived from the observed sunshine duration (SunDu) with reliable accuracy. The SunDu-derived Rs was used as a reference to detect and adjust the inhomogeneity in the observed Rs. However, both the observed Rs and SunDu-derived Rs may have inhomogeneity. In Japan, SunDu has been measured since 1890, and Rs has been measured since 1961 at ~100 stations. In this study, the observed Rs and SunDu-derived Rs were first checked for inhomogeneity with a statistical software RHtest. If confirmed by the metadata of these observations, the detected inhomogeneity was adjusted based on the RHtest-quantile matching method. Second, the two homogenized time series were compared to detect further possible inhomogeneity. If confirmed by the independent ground-based observations of cloud cover fraction, the detected inhomogeneity was adjusted based on the reference dataset. As a result, a sharp decrease in the observed Rs from 1961 to 1975 caused by instrument displacement was detected and adjusted. Similarly, a gradual decline in SunDu-derived Rs due to steady instrument replacement from 1985 to 1990 was detected and adjusted. After homogenization, the two estimates agree well. Rs was found to have increased at a rate of 0.9 W m-2 per decade (p < 0.01) from 1961 to 2015 based on the homogenized SunDu-derived Rs, which was enhanced by a positive aerosol-related radiative effect (2.2 W m-2 per decade) and diminished by a negative cloud cover radiative effect (-1.4 W m-2 per decade). The brightening over Japan was the strongest in spring, likely due to a significant decline in aerosol transported from Asian dust storms. The observed raw Rs data and their homogenized time series used in this study are available at https://doi.org/10.11888/Meteoro.tpdc.271524 (Ma et al., 2021). [ABSTRACT FROM AUTHOR]