The need to develop Si standard solutions as national standards has been expanded, since the importance of Si measurement for evaluating industrial material and environmental analysis is increasing. For developing a Si standard solution, the purity assignment method for high-purity SiO2 using the gravimetric analysis and gravimetric preparation method for Si standard solution was established. Finally, an NMIJ CRM 3645-a Si standard solution was developed. Thirty-three elements in the high-purity SiO2 were measured for impurity evaluation. As a result, the amounts of impurities in SiO2 were low enough not to affect the analytical value obtained by the gravimetric analysis. The purity value of SiO2 was 99.648%. The relative standard deviation (RSD) of the mean value (RSD/√n) was considered the standard uncertainty in the gravimetric analysis, so the final purity value was 99.648% ± 0.074% (k = 2). The stability and homogeneity of the prepared Si standard solution were evaluated by inductively coupled plasma optical emission spectrometry and a continuous flow analyzer. Based on the variability and homogeneity, the standard uncertainties were 0.13% and 0.017%, respectively. Finally, the expanded uncertainty of NMIJ CRM 3645-a Si standard solution was 0.28%.