Recently sun-induced chlorophyll fluorescence (SIF) is used to detect photosynthesis from an ecosystem scale based on physiological mechanisms. We focus on the larch forest which is one of the main ecosystem types in high latitude regions experiencing climate change impact. It is important to know how we can explain SIF signals on different methods and conditions before applying them to the study. In this study, we examined (i) the window size for spectral fitting to retrieve SIF, (ii) the effect of sky cloudiness on the relationship between SIF and photosynthetically active radiation (PAR). We continuously observed upward and downward spectra (ranging 650-800 nm, full width at half maximum < 0.4 nm) on the 32m tower in a Japanese Larch (Larix kampferi) forest at the Fuji-Hokuroku (FHK) flux observation site (35.4 N, 138.8 E) since March 2021. Correlation coefficients between SIF and PAR were varied between clear-sky and overcast-sky in the wide window, although they were relatively consistent between clear-sky and overcast-sky in the narrow window of the spectral fitting. After checking the consistency of SIF detectability, the seasonal variation of SIF implies that depression in the rainy period in late June could be a physiological response in the forest.