Threshold voltage of a CMOS transistor which uses a gate insulator made of a hafnium-based high-k material is optimized. A gate insulator of nMOS and pMOS transistors includes a HfOx film and a HfAlOx film formed thereon. At this time, silicon atoms in a n type polycrystalline silicon film which constitutes a gate electrode and Hf atoms in the HfAlOx film are bonded (Hf—Si bonding) and silicon atoms in the n type polycrystalline silicon film and Al atoms in the HfAlOx film are bonded (Al—O—Si bonding) at the interface between the HfAlOx film and the gate electrode. Consequently, the work function of the n type polycrystalline silicon and the work function of the p type polycrystalline silicon are controlled so as to be symmetrical with respect to a midgap (threshold voltage of MOS transistor=0) by changing the Al concentration in the HfAlOx film.