A nitride semiconductor layer formation method includes the steps of: (S1) placing a substrate in a reaction chamber, the substrate including a −r-plane nitride semiconductor crystal at least in an upper surface; (S2) increasing a temperature of the substrate by heating the substrate placed in the reaction chamber; and (S3) growing a nitride semiconductor layer on the substrate. In the temperature increasing step (S2), a nitrogen source gas and a Group III element source gas are supplied into the reaction chamber.