A semiconductor element built-in device includes: a first substrate having a first pad thereon; a semiconductor element on the first substrate; a second substrate having a second pad thereon and mounted on the first substrate via a solder terminal having a solder coated thereon; a resin layer provided between the first substrate and the second substrate such that the solder terminal and the semiconductor element are embedded in the resin layer; and a dam provided at least partially around at least one of the first and second pads, the dam being configured to restrain the solder flowing from the solder terminal.