In order to validate global horizontal irradiance (GHI) of the weather forecast from the Japan Meteorological Agency Meso-Scale Model (MSM), error causes of relatively large error cases of the forecast GHI in MSM is investigated around Tsukuba, Japan. The extracted 89 cases are categorized as overestimation or underestimation and are investigated in connection with weather conditions. For the case of overestimation, when the thick stratocumulus appears at the low level, the MSM cloud amount forecast fails to simulate such low-level cloud. This is the one of the reason why the forecast GHI becomes overestimation. For the case of underestimation, when the thin clouds appear through the all levels, the MSM fails to simulate cloud amount at middle- and high levels. This is the one of the reason why the forecast GHI becomes underestimated. In order to investigate the relationship between the irradiance forecast and the cloud amount forecast in details, sensitivity experiments of irradiance-related schemes are executed. Two types of experiments are made. One is the sensitivity experiment of the horizontal and vertical resolution in order to change the cloud distribution, which has the strong affection to the forecast GHI. The other is the sensitivity experiment of the effective radius of cloud water and ice affecting the interaction between the cloud and the solar radiation. Since both experiments have strong sensitivity to the forecasted GHI, it is important to develop the scheme which forecasts the cloud distribution and to choose the suitable effective radius of cloud water and ice.