A pattern forming method includes (a) forming pairs of deposits on sidewalls of mask portions in first mask patterns by forming a thin film thereon, etching it to leave deposits, and exposing a top surface of a second-layer film between the deposits; (b) forming second mask patterns formed of mask portions corresponding to the deposits by removing the mask portion, plasma etching the second-layer film, and removing the deposits; (c) forming a thin film thereon, and etching it to leave deposits on sidewalls of mask portions facing each other and to expose a third-layer film between the deposits while leaving deposits between adjacent mask portions; and (d) forming grooves thereon by removing the second mask portion, and etching off the third-layer film.