NOVELTY - Forming an impurity diffusion layer comprises applying a diffusing agent composition (2) to a semiconductor substrate (1), thus forming a pattern. The diffusing agent composition is used to print an impurity diffusion component onto a semiconductor substrate, and includes a condensation product made from a starting material that is an alkoxysilane (I) and an impurity diffusion component. The impurity diffusion component (b) is a phosphate (II). The method further comprises diffusing the impurity diffusion component of the diffusing agent composition into the semiconductor substrate.    USE - The method is useful for forming an impurity diffusion layer in a semiconductor substrate which is useful in a solar cell (all claimed).    ADVANTAGE - The method utilizes the diffusing agent composition which: contains 0% or 1 mass% at most of water (all claimed); has improved storage stability and simultaneously improved diffusibility of the impurity diffusion component; exhibits improved paintability, flattenability and developability; and reduces the generation of unevenness in the painted layer-tissue.    DETAILED DESCRIPTION - Forming an impurity diffusion layer comprises applying a diffusing agent composition (2) to a semiconductor substrate (1), thus forming a pattern. The diffusing agent composition is used to print an impurity diffusion component onto a semiconductor substrate, and includes a condensation product made from a starting material that is an alkoxysilane of formula (I) and an impurity diffusion component. The impurity diffusion component is a phosphate of formula (II). The method further comprises diffusing the impurity diffusion component of the diffusing agent composition into the semiconductor substrate.    R1, R2 = organic group;    R3 = alkyl;    m = 0-2;    n = 4-m;    p = 1 or 2; and    q = 3-p.    DESCRIPTION OF DRAWING(S) - The figure shows a process sectional view of a solar-cell-producing process by forming an impurity diffusion layer.    Semiconductor substrate (1)    Diffusing agent composition (2, 3)    P-type impurity diffusion layer (4)    P-type impurity diffusion layer (5)