We analyzed a long-term increase in solar radiation and its relation to primary production in a shallow, eutrophic lake. Solar radiation is increasing globally, and previous study has reported that solar radiation at Lake Kasumigaura is increasing during the springtime. We tested the hypothesis that increase in the springtime solar radiation also increases primary production because (1) phytoplankton biomass is the highest during the springtime; and (2) the water temperature also increased. We have been monitoring primary production at 4 sampling stations and constructed a model (Production–Irradiance [P–I] curve) for the analysis. We estimated that primary production increased by 13.2% (0.093 gC m–2 h–1~0.105 gC m–2 h–1) from 1992 to 2019 due to the combined effects of the increases in the water temperature increase and solar radiation. However, nutrient effects, i.e. POC:PON ratio, are much greater than the effects of the increases in the water temperature and solar radiation. These results suggest that, although nutrient effects are larger than the combined effects of solar radiation and water temperatures, primary production is affected by the increase in solar radiation in Lake Kasumigaura.