A semiconductor device includes a wiring substrate having a mounting surface on which a semiconductor element is mounted. A portion of the mounting surface exposed from the semiconductor element is covered by a solder-resist layer, and an extension portion of the solder-resist layer extends from a dropping-commencing point of a liquid-state under-filling agent on the portion of the mounting surface exposed from the semiconductor element and into an area of the wiring substrate covered by the semiconductor element. A gap between the semiconductor element and the extension portion of the solder-resist layer is formed to be narrower than the gap between the semiconductor element and the mounting surface of the wiring substrate so that liquid drops of the under-filling agent dropped at the dropping-commencing point are sucked into the gap by a capillary phenomenon.