A method of manufacturing a semiconductor device includes: coating a first resist containing a photoacid generator or a thermal acid generator on a semiconductor substrate; forming a first opening portion in the first resist by optical exposure; subjecting a shrink agent containing an acid to a crosslinking reaction by the heat treatment to form a thermoset layer on an overall surface of the first resist; coating a second resist on the semiconductor substrate and the thermoset layer; and forming a second opening portion located above the first opening portion and larger than the first opening portion in the second resist by optical exposure.