PURPOSE: To adjust a sectional area completely by driving an auxiliary nozzle vertically according to the degree of traveling speed inside a main nozzle where a rear end is reduced in its diameter in the case of changing the sectional area of jet stream jetted from a nozzle according to traveling speed.

CONSTITUTION: Water sucked in from a boat bottom is pressurized by a pump impeller 3, and after it is rectified by a guide impeller 5, it is jetted as jet stream from the nozzle 6 of a stern to obtain thrust. The sectional area of jet stream jetted from the nozzle 6 is changed by a jet stream adjusting means 21 according to traveling speed, whereas a jet stream adjusting means 21 consists of an auxiliary nozzle 23 which can move freely in an axial direction on the downstream side of a main nozzle 22 where its rear end is reduced in diameter and a driving mechanism 24 which drives the auxiliary nozzle 23 to downstream side at the time of high speed and to upstream side at the time of low speed respectively. The driving mechanism 24 is controlled by a controller 27 through a driving device 28 according to traveling speed.