A semiconductor device having a non-volatile memory is disclosed, whose disturb defect can be diminished or prevented. A memory cell of the non-volatile memory has a memory gate electrode formed over a main surface of a semiconductor substrate through an insulating film for charge storage. A first side wall is formed on a side face of the memory gate electrode, and at a side face of the first side wall, a second side wall is formed. On an upper surface of an n+-type semiconductor region for source in the memory cell there is formed a silicide layer whose end portion on the memory gate electrode MG side is defined by the second side wall.