A GaP light emitting element substrate comprising an n-type GaP layer, a nitrogen-doped n-type GaP layer and a p-type GaP layer layered one after another on a multi-layer GaP substrate built by forming an n-type GaP buffer layer(s) on an n-type GaP single crystal substrate, wherein the sulfur (S) concentration in said n-type GaP buffer layer is made to be 5.times.10.sup.16 [atoms/cc] or less. The method of manufacturing it is as follows: an n-type GaP buffer layer(s) is formed on an n-type GaP single crystal substrate to prepare a multi-layer GaP substrate, then an n-type GaP layer, a nitrogen doped n-type GaP layer and a p-type GaP layer are layered on said multi-layer GaP substrate by means of the melt-back method to obtain a GaP light emitting element substrate, wherein the sulfur (S) concentration in said n-type GaP buffer layer is made to be 5.times.10.sup.16 [atoms/cc] or less when the multi-layer GaP substrate is prepared.