To decrease the occurrence of frictional resistance of the bottom of a small planing boat through simple constitution, to increase a maximum ship speed, to improve acceleration performance, and to suppress the occurrence of a side slip during turning.

A discharge port 4 through which gas is discharged in water is formed in the part, forming a water surface, of the bottom of a small planing boat. Gas is guided to the discharge port 4 by means of a discharge duct 5. By locating gas, discharged through the discharge port 4, between the bottom and an external water flow, frictional resistance at the bottom is reduced, a maximum ship speed is increased, and acceleration performance is improved. A control valve 59 to control a flow rate of gas passing through the discharge duct 5 is located in the discharge duct 5. The opening of the control valve 59 is regulated by a valve drive means 60 operated in linkage with a steering device 58, and the opening of the control valve 59 is decreased according to the increase of a rotation amount of a steering device 58. This constitution suppresses the occurrence of a side slip during turning.