An apparatus for forming a plastic fuel tank for a vehicle, the apparatus forming a parison into semi-finished products having a shape of the fuel tank and forming the semi-finished products into a finished plastic fuel tank, the apparatus comprising: first and second molds configured to be combined and separated from each other; and an intermediate mold configured to be inserted between and withdrawn from the first and second molds, wherein when the parison is formed into the semi-finished products, the intermediate mold is positioned inside the parison and serves as a core, and when the first and second molds are combined, the first and second molds press an outer surface of the parison and the intermediate mold supports an inner surface of the parison, whereby the parison is formed into the semi-finished products, wherein the intermediate mold includes: a body portion inserted into cavities of the first and second molds when the first and second molds are combined; and a flange portion protruding outwardly from the body portion and positioned between the first and second molds at a position outside the cavities when the first and second molds are combined, and the apparatus being characterised by further comprising: a plurality of air passages provided at the first and second molds, and communicating with the cavities of first and second molds; an air pump connected to the air passages through a plurality of air lines; and control valves provided on the air lines, respectively, and configured to allow the air passages and the air pump to be connected to each other and to allow the air passages and the atmosphere to communicate with each other in accordance with control of a controller. first and second molds configured to be combined and separated from each other; and an intermediate mold configured to be inserted between and withdrawn from the first and second molds, wherein when the parison is formed into the semi-finished products, the intermediate mold is positioned inside the parison and serves as a core, and when the first and second molds are combined, the first and second molds press an outer surface of the parison and the intermediate mold supports an inner surface of the parison, whereby the parison is formed into the semi-finished products, wherein the intermediate mold includes: a body portion inserted into cavities of the first and second molds when the first and second molds are combined; and a flange portion protruding outwardly from the body portion and positioned between the first and second molds at a position outside the cavities when the first and second molds are combined, and the apparatus being characterised by further comprising: a plurality of air passages provided at the first and second molds, and communicating with the cavities of first and second molds; an air pump connected to the air passages through a plurality of air lines; and control valves provided on the air lines, respectively, and configured to allow the air passages and the air pump to be connected to each other and to allow the air passages and the atmosphere to communicate with each other in accordance with control of a controller. a body portion inserted into cavities of the first and second molds when the first and second molds are combined; and a flange portion protruding outwardly from the body portion and positioned between the first and second molds at a position outside the cavities when the first and second molds are combined, and the apparatus being characterised by further comprising: a plurality of air passages provided at the first and second molds, and communicating with the cavities of first and second molds; an air pump connected to the air passages through a plurality of air lines; and control valves provided on the air lines, respectively, and configured to allow the air passages and the air pump to be connected to each other and to allow the air passages and the atmosphere to communicate with each other in accordance with control of a controller. a plurality of air passages provided at the first and second molds, and communicating with the cavities of first and second molds; an air pump connected to the air passages through a plurality of air lines; and control valves provided on the air lines, respectively, and configured to allow the air passages and the air pump to be connected to each other and to allow the air passages and the atmosphere to communicate with each other in accordance with control of a controller.