In a semi-molten metal injection molding method of producing a thin molded product by injecting a semi-molten metal M of a magnesium alloy, in a semi-melting state, into a cavity of a mold through a product gate, characterized in that it is made possible to obtain a high-quality thin molded product by maintaining satisfactory fluidity of the semi-molten metal M. A grain size of the solid fraction in the melt M is set to not more than 0.13 times the average thickness of the product portion of the thin molded product and a molten metal velocity at the product gate is set to not less than 30 m/s and, moreover, a solid fraction Fs (%) of the semi-molten metal M and a grain size D (&mgr;m) of the solid phase of the semi-molten metal M are set so as to define the relationship Fs×D≦1500.