Disclosed is a semiconductor integrated circuit device having a plurality of fine memory devices and its fabrication method, and particularly to a semiconductor integrated circuit device capable of suppressing the kink current disturbance of MOS transistors without reducing the junction characteristic of the diffusion layers and its fabrication method. In this device, an angle between the lower surface of each edge of a field oxide formed in an environmental device area, i.e. a peripheral circuit area, and the main surface of a semiconductor substrate is smaller than an angle between the lower surface of each edge of a field oxide formed in a memory cell area and the main surface of the semiconductor substrate. Further, the extension, in the direction of being parallel to the main surface of the semiconductor substrate, of each edge of the field oxide in the environmental device area is larger than the extension, in the direction of being parallel to the main surface of the semiconductor substrate, of each edge of the field oxide in the memory cell area.