A method for manufacturing a semiconductor device by forming a semiconductor package with a heat dissipating pad, a semiconductor element mounted on the heat dissipating pad, electrodes disposed at a distance from the heat dissipating pad and electrically connected to the semiconductor element, and a sealant resin body for sealing at least the semiconductor element, and joining the heat dissipating pad and the electrodes separately to a substrate via a solder layer, which comprises: a first step of producing a semiconductor package intermediate by injecting a resin used for the sealant resin body into a forming die in which the electrodes, the heat dissipating pad, and the semiconductor element are disposed, providing a peel-off film on one side of the resin, the resin being in the form of a still-uncured resin body, opposite from the other side facing the heat dissipating pad and a rigid material on one side of the peel-off film opposite from the other side facing the uncured resin body, and curing the uncured resin body to form the sealant resin body; a second step of forming the solder layer by reflow soldering between the substrate and each of the heat dissipating pad and the electrodes that constitute the semiconductor package intermediate removed from the forming die in the first step; and a third step of removing the rigid material from the peel-off film, wherein the rigid material is integrated into the semiconductor package intermediate so as to make the thermal expansion coefficient and rigidity of the semiconductor package intermediate approximately equal to those of the substrate, by selecting the rigid material having a higher degree of rigidity and lower thermal expansion coefficient than the sealant resin body as a constituent of the semiconductor package intermediate, and projections are formed on one side of the rigid material facing the peel-off film so as to allow the projections to be embedded in the uncured resin body when the rigid material is disposed on one side of the uncured resin body. a first step of producing a semiconductor package intermediate by injecting a resin used for the sealant resin body into a forming die in which the electrodes, the heat dissipating pad, and the semiconductor element are disposed, providing a peel-off film on one side of the resin, the resin being in the form of a still-uncured resin body, opposite from the other side facing the heat dissipating pad and a rigid material on one side of the peel-off film opposite from the other side facing the uncured resin body, and curing the uncured resin body to form the sealant resin body; a second step of forming the solder layer by reflow soldering between the substrate and each of the heat dissipating pad and the electrodes that constitute the semiconductor package intermediate removed from the forming die in the first step; and a third step of removing the rigid material from the peel-off film, wherein the rigid material is integrated into the semiconductor package intermediate so as to make the thermal expansion coefficient and rigidity of the semiconductor package intermediate approximately equal to those of the substrate, by selecting the rigid material having a higher degree of rigidity and lower thermal expansion coefficient than the sealant resin body as a constituent of the semiconductor package intermediate, and projections are formed on one side of the rigid material facing the peel-off film so as to allow the projections to be embedded in the uncured resin body when the rigid material is disposed on one side of the uncured resin body.