To improve reliability of a semiconductor device, a control transistor and a memory transistor formed in a memory cell region are configured to have a double-gate structure, and a transistor formed in a peripheral circuit region is configured to have a triple-gate structure. For example, in the memory transistor, a gate insulating film formed by an ONO film is provided between a memory gate electrode and sidewalls of a fin, and an insulating film (a stacked film of a multilayer film of an insulating film/an oxide film and the ONO film) thicker than the ONO film is provided between the memory gate electrode and a top surface of the fin. This configuration can reduce concentration of an electric field onto a tip of the fin, so that deterioration of reliability of the ONO film can be prevented.