A resin-seal type semiconductor device includes a main substrate divided into first and second surfaces on which first and second insulating substrates are respectively provided. Circuit wiring patterns and first end portions of outer leads are provided on the first and second insulating substrates. Semiconductor elements are soldered onto the wiring patterns and coated with a surface protection material. The main substrate is then folded over so that the first and second surfaces oppose one another and the device acquires a U-shaped cross-section. A space formed between the first and second surfaces inside the U-shape is sealed with a resin such that second end portions of the outer leads are exposed from the resin.