In a split-gate MONOS memory including a FINFET, occurrence of erroneous write in an unselected cell due to electric field concentration at an upper end of a fin is prevented, and thus reliability of a semiconductor device is improved. An insulating film is formed between an upper surface of a fin and each of a control gate electrode and a memory gate electrode in a memory cell region, so that in a gate insulating film of each of a control transistor and a memory transistor, thickness of a portion on the fin is larger than thickness of a portion covering side surfaces of the fin. The insulating film having a bird's beak at its end portion is formed to round a corner of the fin.