A semiconductor device includes a semiconductor substrate, p-type first and n-type second semiconductor regions formed on the substrate so as to be insulated with each other, n-channel and p-channel MOS transistors formed on the first and second semiconductor regions, the n-channel transistor including a first pair of source/drain regions formed on the first semiconductor region, a first gate insulator formed in direct contact with the first semiconductor region and formed as an amorphous insulator containing at least La, and a first gate electrode formed on the first gate insulator, the p-channel MOS transistor including a second pair of source/drain regions formed opposite to each other on the second semiconductor region, a second gate insulator including a silicon oxide film and the amorphous insulating film formed thereon on the second semiconductor region, and a second gate electrode formed on the second gate insulator.