It is disclosed an over-coating agent for forming fine patterns which is applied to cover a substrate having photoresist patterns thereon and allowed to shrink under heat so that the spacing between adjacent photoresist patterns is lessened, with the applied film of the over-coating agent being removed to form fine patterns, further characterized by comprising a water-soluble polymer which contains a monomeric component and a dimeric component, wherein the total content of the monomeric component and the dimeric component in the water-soluble polymer is reduced to 10 mass % or less, and a method of forming fine patterns using the same. By the present invention, even in reducing the pattern size on a substrate having thereon patterns having different pitches, the heat shrinkage of the over-coating agent can be controlled, irrespective whether the pitch is dense or isolate, thus achieving the pattern size reduction.