A method of manufacturing an integrated circuit (IC) device includes exposing a partial region of a photoresist film formed on a main surface of a substrate to generate acid, and diffusing the acid in the partial region of the photoresist film. Diffusing the acid may include applying an electric field, in a direction perpendicular to a direction in which the main surface of the substrate extends, to the photoresist film using an electrode facing the substrate through an electric-field transmission layer filling between the photoresist film and the electrode. The electric-field transmission layer may include an ion-containing layer or a conductive polymer layer.