A light metal injection molding machine includes an injection cylinder, a seal cylinder, a plunger, and a seal mechanism having a first annular body, a second annular body and a third annular body. The first annular body is arranged in an opening of the seal cylinder, and maintains a semisolid state of a molding material in a first gap between its inner periphery surface and the plunger. The second annular body is arranged in the opening between the injection cylinder and the first annular body, and includes an anterior internal groove formed over its whole inner periphery surface and a plurality of anterior transverse holes passing through the inner periphery surface. The third annular body is accommodated into the anterior internal groove transformably in a radial direction of the plunger, and tighten the plunger by pressurization from molding material flowing into the anterior transverse holes.