A method of manufacturing a semiconductor device having a memory cell for a split-gate MONOS memory with a halo region, which prevents miswriting in the memory cell and worsening of short channel characteristics. In the method, a first diffusion layer of a drain region and a second diffusion layer of a source region in the memory cell for the MONOS memory are formed in different ion implantation steps. The steps are carried out so that the first diffusion layer has a smaller formation depth than the second diffusion layer. After the formation of the layers, the impurities inside the first and second diffusion layers are diffused by heat treatment to form a first diffusion region and a second diffusion region.