A boron phosphide-based compound semiconductor device with excellent device properties, comprising a boron phosphide-based compound semiconductor layer having a wide bandgap is provided. The boron phosphide-based compound semiconductor layer consists of an amorphous layer and a polycrystal layer provided to join with the amorphous layer, and the room-temperature bandgap of the boron phosphide-based compound semiconductor layer is from 3.0 eV to less than 4.2 eV.