To increase the percentage of voids near the surface of a hull shell for effectively reducing friction resistance.

A number of air blow portions 12 is provided on the outer periphery of a bulb end member 11, spaced from each other as required in the peripheral direction. The air blow portions 12 are provided with air blow ports 15 directed almost to a stream line for connecting thereto an air chamber 13 provided inside the bulb end member 11 via an air conduit 14. The bulb end member 11 is welded to a hull 1 in one unit, independently from the hull 1, to form a bow bulb 1b. A compressor 7 is connected to the air chamber 13 via an air feed pipe 9. Pressure air 10 generated by the compressor 7 is blown from the air blow ports 15 into water and small bubbles 10a are filled into a thin portion of a boundary layer to form voids in the surface of a dipped portion of the hull 1.