A technique is provided which permits formation within a single chip both a field effect transistor of high reliability capable of suppressing the occurrence of a crystal defect and a field effect transistor of a high integration degree. In a mask ROM section having an element isolation region with an isolation width of smaller than 0.3 .mu.m, a planar shape of each active region ACT is made polygonal by cutting off the corners of a quadrangle, thereby suppressing the occurrence of a crystal defect in the active region ACT and diminishing a leakage current flowing between the source and drain of a field effect transistor. In a sense amplifier data latch section which is required to have a layout of a small margin in the alignment between a gate G of a field effect transistor and the active region ACT, the field effect transistor is disposed at a narrow pitch by making the active region ACT quadrangular.