Abstract: The present invention provides a boron diffusion layer forming method capable of sufficiently oxidizing a boron silicide layer formed on a silicon substrate to remove it and obtaining a high-quality boron silicate glass layer. The present invention is a boron diffusion layer forming method of forming a boron diffusion layer on a silicon substrate by a boron diffusion process, the process including a first step of thermally diffusing boron on the silicon substrate and a second step of oxidizing a boron silicide layer formed on the silicon substrate at the first step, wherein the second step has a state at a temperature of 900° C. or higher and a treatment temperature at the first step or lower, for 15 minutes or more.