A method of manufacturing a semiconductor device according to an embodiment of the present invention includes depositing first to third mask layers above a substrate, processing the third mask layer, processing the second mask layer, slimming the second mask layer in an L/S section and out of the L/S section, peeling the third mask layer in the L/S section and out of the L/S section, forming spacers on sidewalls of the second mask layer in the L/S section and out of the L/S section, etching the second mask layer in the L/S section, under a condition that the second mask layer out of the L/S section is covered with a resist, to remove the second mask layer in the L/S section while the second mask layer out of the L/S section remains, and processing the first mask layer by etching, using the spacers in the L/S section and out of the L/S section and the second mask layer out of the L/S section as a mask, the spacers in the L/S section and out of the L/S section and the second mask layer out of the L/S section being thinned by the etching.