When upper surfaces of a control gate electrode and a memory gate electrode are exposed from an interlayer insulating film by polishing the interlayer insulating film in a gate last process, a silicide layer covering the upper surfaces of the gate electrodes is formed. Thereafter, by reacting a metal film deposited on the silicide layer with the control gate electrode and the memory gate electrode, a silicide layer thicker than the former silicide layer is formed on each of the gate electrodes.