Abstract: An airtight device for a container cap includes a container having a pouring portion defined therein. A filling tube is selectively inserted into the container via the pouring portion. The filling tube has an annular flange radially and outwardly extending from one end thereof. An airtight element is air-tightly mounted to the annular flange of the filling tube. A cap is mounted to the pouring portion in the container. A through hole is defined in the cap to allow the filling tube to extend through the cap during filling. A shelter mounted between the annular flange and the cap for closing the through hole in the cap after filling. The airtight device is conveniently to fill oil from the container because the operator only needs to inversely mount the filling tube such that the palms of the operator will not be soiled during mounting the filling tube.