There are provided a method of manufacturing a semiconductor device which is capable of narrowing only the width of a Fin channel while maintaining the widths of source and drain regions, and a semiconductor device. The method of manufacturing a semiconductor device is a method of manufacturing a Fin type transistor, including: forming STI region 2 with use of mask layer 8 formed over silicon substrate 1 as a mask; narrowing mask layer 8 by wet etching to form narrowed mask layer 8a; forming stopper oxide film 16 over a surface of narrowed mask layer 8a, depositing polysilicon 17 over an entire surface and then forming anti-reflective film 18 and photoresist 19; forming an opening at photoresist 19 in a portion corresponding to a word line portion, removing anti-reflective film 18 and polysilicon 17 in that portion to expose narrowed mask layer 8a and then removing photoresist 19; and forming Fin channel 30 by etching portions of silicon substrate 1 which lie on opposite sides of and below narrowed mask layer 8a with narrowed mask layer 8a as a mask.