Affords a method of growing, across the entirety of a major surface of a first III-nitride crystal, a second III-nitride crystal by HVPE, in an ambient temperature higher than 1100° C. The present III-nitride crystal growth method comprises: a step of preparing a first III-nitride crystal (10) having an alkali-metal atom concentration of less than 1.0×1018 cm−3; and a step of growing a second III-nitride crystal (20) onto a major surface (10m) of the first III-nitride crystal (10) by HVPE, in an ambient temperature higher than 1100° C.