The present invention relates to a method of manufacturing a semiconductor device having an excellent gettering effect. In this method, when phosphorus is added to a poly-Si film, which has been crystallized by the addition of a metal, to subject the resultant poly-Si film to the heat treatment to carry out gettering therefor, the device is performed for the shape of the island-like insulating film on the poly-Si film which is employed when implanting phosphorus. Thereby, the area of the boundary surface between the region to which phosphorus has been added and the region to which no phosphorus has been added is increased to enhance gettering efficiency.