A semiconductor device manufacturing method of an embodiment includes the steps of: forming a first insulating layer on a semiconductor substrate; forming on the first insulating layer an amorphous or polycrystalline semiconductor layer having a narrow portion; forming on the semiconductor layer a second insulating layer having a thermal expansion coefficient larger than that of the semiconductor layer; performing thermal treatment; removing the second insulating layer; forming a gate insulating film on the side faces of the narrow portion; forming a gate electrode on the gate insulating film; and forming a source-drain region in the semiconductor layer.