PURPOSE: To keep off any vibration of a flap in a steering cylinder in particular as well as to promote the stabilization of a flap driving system and the prevention of a separation noise of flowing in the flap, in regard to a marine water jet adopted type steering cylinder device made up of performing each of forward, neutral holding and backward movements owing to this steering cylinder where a water jet is adopted.

CONSTITUTION: In a steering cylinder 2 being installed in the rearward of a propelling water jet injection nozzle 1 and having a rear opening in the rear and a bottom opening in the bottom respectively, a flap 5 is provided which is installed therein and makes the rear opening or bottom opening closable in rotation, and plural pieces of small holes are installed in place nearby a tip of the flap 5. With this constitution, in an intermediate rising state 5' of the flap 5, a flow of water jetting from the nozzle 1 flows through the inner part of a small hole 5'a, so that exciting force against the flap 5 and a separation noise of flowing are made so as to be restrained from occurring.