In order to examine the effect on the insolation of cloud cover, we analyzed the data of the insolation, cloud (over and surface air pressure in Pusan during the period of 1991. 10 - 1993.1. At first, we investigate the atmospheric transmissivity A(t) using the Beer`s law at clear skies. The atmospheric transmissivity is characterized by cold season high and warm season low. From this atmospheric transmissivity, the empirical formula that shows the variation of the insolation due to the cloud cover is obtained. The result formula is I = I_0 A(t^n)(0.7 - 0.05 × m ). I is the insolation that reaches the surface when cloud cover is m and I_0 is solar constant. Although the result is some rough it seems meaningful that the estimation of insolation can be made only from the routine data.