A horizontal molding apparatus of an interior material of a vehicle, the horizontal molding apparatus comprising: - a preheating means for preheating a skin material; - a transfer unit (60;70) formed to be movable and comprising fixing units (62;72) for vertically attaching or detaching the preheated skin material on or from the transfer unit (60;70); - a first mold disposed perpendicularly to a ground surface and comprising suction units for vacuum-sucking and temporarily fixing edges of the skin material transferred by the transfer unit (60;70), a base member having an embossed pattern to be printed on the skin material, and vacuum holes for vacuum-sucking the skin material onto the base member; - a second mold disposed in parallel with the first mold and on which a base material is injected; and - a mold moving means for horizontally moving one of the first and second molds and combining or separating the first and second molds to or from each other, characterized by a) fixing members for sealing an internal space between the skin material and the base member by clamping and completely fixing onto the first mold the edges of the skin material, which are temporarily fixed by suction units, wherein the fixing members comprise: - clamps for supporting the edges of the skin material, the clamps comprising an L-shaped cross section with two arms, one parallel to the plane of the skin material, and the other perpendicular to such plane; - vertical cylinders for vertically moving the clamps; and - horizontal cylinders for horizontally moving the vertical cylinders; and b) further comprising an air nozzle for injecting air into the sealed internal space between the skin material and the base member. - a preheating means for preheating a skin material; - a transfer unit (60;70) formed to be movable and comprising fixing units (62;72) for vertically attaching or detaching the preheated skin material on or from the transfer unit (60;70); - a first mold disposed perpendicularly to a ground surface and comprising suction units for vacuum-sucking and temporarily fixing edges of the skin material transferred by the transfer unit (60;70), a base member having an embossed pattern to be printed on the skin material, and vacuum holes for vacuum-sucking the skin material onto the base member; - a second mold disposed in parallel with the first mold and on which a base material is injected; and - a mold moving means for horizontally moving one of the first and second molds and combining or separating the first and second molds to or from each other, a) fixing members for sealing an internal space between the skin material and the base member by clamping and completely fixing onto the first mold the edges of the skin material, which are temporarily fixed by suction units, wherein the fixing members comprise: - clamps for supporting the edges of the skin material, the clamps comprising an L-shaped cross section with two arms, one parallel to the plane of the skin material, and the other perpendicular to such plane; - vertical cylinders for vertically moving the clamps; and - horizontal cylinders for horizontally moving the vertical cylinders; and - clamps for supporting the edges of the skin material, the clamps comprising an L-shaped cross section with two arms, one parallel to the plane of the skin material, and the other perpendicular to such plane; - vertical cylinders for vertically moving the clamps; and - horizontal cylinders for horizontally moving the vertical cylinders; and b) further comprising an air nozzle for injecting air into the sealed internal space between the skin material and the base member.