To maintain tooth contact of a forward gear and a driving gear in a proper state, even if the big load is applied to the forward gear, in a reversing mechanism of the outboard motor which has the driving gear and the forward gear engaging therewith.

A support structure of the gear of the outboard motor of a counter rotation specification has: a driving gear that rotates by the rotation power transmitted from the engine; a forward gear 52 that has a cylindrical structure that penetrates the back and forth direction and is disposed at a rear side of the driving gear; a propeller shaft 29 to which the propulsive propellers are attached and rotatably disposed penetrating through the forward gear 52; a tapered roller bearing 34 provided between the inner periphery of forward gear 52 and the periphery of propeller shaft 29; and a precompression giving member 54 that provides the backward precompression to the forward gear 52.