The invention provides a method of manufacturing a semiconductor device which achieves high reliability and high yield as well as high production efficiency. Back surface grinding (back grinding) is performed to a semiconductor substrate to thin the semiconductor substrate. A damaged layer formed by the back surface grinding is not removed at this time, and a photoresist layer is selectively formed on the back surface of the semiconductor substrate. The semiconductor substrate is then etched using the photoresist layer as a mask to form a via hole. The photoresist layer is then removed with the semiconductor substrate still placed in an etcher used in the etching process subsequently after the formation of the via hole. In this manner, the etching process and the next ashing process are performed sequentially in one apparatus. Then a process of removing the damaged layer on the back surface of the semiconductor substrate and a process of smoothing the sidewall of the via hole are simultaneously performed subsequently after the ashing process in the same apparatus.