The present invention relates to a watertight sealing device for a portion of a pipeline connected to a wall of a structure which facilitates watertight sealing work on a previously installed pipeline by divided flanges and increases a cross sectional area of a sponge rubber ring to reliably guarantee watertightness. The watertight sealing device for a portion of a pipeline connected to a wall of a structure comprises: a guide ring which is installed in a watertight manner through a hole penetrating the wall of the structure to pass the pipeline thereinto, and has an inner diameter relatively larger than an outer diameter of the pipeline to form a gap between the inner diameter thereof and the outer diameter of the pipeline; a circular flange fixed on one end of the guide ring and provided with bolt holes formed at regular intervals; a pair of divided flanges in a divided form corresponding and facing the circular flange and having bolt holes corresponding to the bolt holes; and a sponge rubber ring disposed between the circular flange and the divided flanges and tightly attached to the pipeline by tightening bolts/nuts of the circular flange and the divided flanges to seal the pipeline in a watertight manner.
