The fabricating method for semiconductor devices in which the trench technique is employed to perform isolation between devices, and which comprises the steps of sequentially depositing a first film 2, 3 and a second film 4 on top of a silicon substrate 1, forming an element isolation trench 5 in the silicon substrate 1 with masking of the first film 2, 3 and second film 4 which have undergone patterning, and growing a silicon oxide film 6 that is generated by reaction of ozone and tetra-ethyl-ortho-silicate inside the element isolation trench where silicon is exposed.