A method for manufacturing a semiconductor substrate that, even when a substrate which has, on a surface thereof, a three-dimensional structure having nanometer-scale microvoids on a surface thereof is used, can allow an impurity diffusion ingredient to be uniformly diffused into the substrate at the whole area thereof where the diffusion agent composition is coated, including the whole inner surfaces of the microvoids, while suppressing the occurrence of defects in the substrate. A coating film having a thickness of not more than 30 nm is formed on a surface of a substrate under such conditions that an atmosphere around the substrate has a relative humidity of not more than 40%, using a diffusion agent composition comprising an impurity diffusion ingredient and a Si compound that is hydrolyzable to produce a silanol group.