There is provided a method for manufacturing a group III nitride substrate, including: preparing a plurality of seed crystal substrates formed into shapes that can be arranged with side surfaces opposed to each other; bonding the plurality of seed crystal substrates on a base material by an adhesive agent in an appearance that the seed crystal substrates are arranged with the side surfaces opposed to each other; growing a group III nitride crystals above main surfaces of the plurality of seed crystal substrates, so that crystals grown on each main surface are integrally combined each other; and obtaining a group III nitride substrate formed of the group III nitride crystal.