A method for forming a quantum well structure that can reduce the variation in the In composition in the thickness direction of a well layer and a method for manufacturing a semiconductor light emitting element are provided. In a step of forming a quantum well structure (active layer) by alternately growing barrier layers and well layers on a primary surface of a GaN substrate, the well layers are each formed by growing InGaN, the barrier layers are each grown at a first temperature, the well layers are each grown at a second temperature which is lower than that of the first temperature, and when the well layers are each formed, before a starting material gas for Ga (trimethylgallium) is supplied, a starting material gas for In is supplied.