A semiconductor device includes: a mounted body in which a wiring pattern is formed on a first main surface; a semiconductor chip mounted on the surface of the mounted body on which the wiring pattern is formed; an underfill material which is filled between the mounted body and the semiconductor chip and forms a fillet on an outer peripheral part of the semiconductor chip; and an injection section which is disposed on the mounted body and on an outside of a side section, on which the fillet is formed to be longest, of four side sections defining a chip mount area on which the semiconductor chip is mounted, and guides the underfill material to between the mounted body and the semiconductor chip.