It is to provide a substrate for growing a semiconductor, which is effective for suppressing an occurrence of surface defects different in type from hillock defects in case of epitaxially growing a compound semiconductor layer, particularly an Al-based compound semiconductor layer.In a substrate for growing a compound semiconductor, in which a crystal surface inclined at a predetermined off angle with respect to a (100) plane is a principal plane, an angle made by a direction of a vector obtained by projecting a normal vector of the principal plane on the (100) plane and one direction of a [0-11] direction, a [01-1] direction, a [011] direction and a [0-1-1] direction is set to be less than 35°, and the compound semiconductor layer is epitaxially grown on the substrate.