Assessment of the energy saving performance of high performance glass, selective coatings, and photovoltaic power generation systems in buildings requires data, the annual spectral solar irradiance under all weather conditions. Existing annual hourly weather data include the amount of solar irradiation measured with pyranometers, but there is a lack of spectral solar irradiance data. Thus, for the results to be meaningful, a model of spectral global solar irradiance and a model of spectral direct or spectral diffuse solar irradiance are necessary to supplement the existing hourly weather data.<br> In this paper, we develop a simple regression formula that estimates spectral diffuse solar irradiance incident on a horizontal surface from four weather parameters, such as diffuse solar irradiation, global solar irradiation, air temperature, and humidity.<br> Using grating spectroradiometers (MS-710 and MS-712) with rotating shadow blade (PRB-100) to record the irradiance between 350 and 1700 nm, we measured the spectral global solar irradiance and the spectral diffuse solar irradiance under a variety of weather conditions for two years, from 2010 to 2011. Simultaneously, we also used a pyranometer (MS-802), conforming to the ISO 9060 Secondary Standard, to measure the amount of diffuse solar irradiation between 305 and 2800 nm. These measurements were conducted at Kagoshima city, Japan.<br> Using the measurements from 2010, we found a high degree of correlation between the spectral diffuse solar irradiance and the amount of diffuse solar irradiation.<br> We then created a regression formula that estimates spectral diffuse solar irradiance from 350 to 1700 nm at 5 nm intervals. In this regression formula, the amount of diffuse solar irradiation is used as an independent variable. We also found that the coefficients of this regression formula vary depending on the wavelength of the solar irradiance, clearness index, air mass, and precipitable water in the atmosphere. Using these variables we divided the measurements into 36 datasets, and using the least squares method we determined the coefficients of this formula on 36 weather conditions. We then used Equation (5) with these derived coefficients as the equation to estimate spectral diffuse solar irradiance.<br> Using Equation (5), we calculated the all-weather spectral diffuse solar irradiance during 2011 and compared it with the measured values. The root mean square error for the estimated spectral diffuse solar irradiance with respect to the measured values was found to be 0.1～30.7 [W/m²/μm] at 350～1700 nm.