To provide a resistance reducing device of a vessel preventing lowering of efficiency of a propeller and vibration of a hull due to inflow of bubbles into the propeller.

This resistance reducing device 20 of the vessel includes an air blowing device 30 for blowing air into water from a draft lower part of a bow side part 11 of the hull 10, and a stern addition 41 provided in a stern side part 12 of the hull 10 to straighten and accelerate a water flow of an upper part of the propeller 16. The water flow of the upper part of the propeller 16 is straightened and accelerated by the stern addition 41, and thereby bubbles of air layers 200 and 201 are prevented from staying in the vicinity of the propeller 16. As a result, the lowering of efficiency of the propeller 16 and vibration of the hull 10 due to inflow of the bubbles into the propeller 16 are prevented.