The invention concerns a housing (1) for an angle gear box device of a boat propulsion motor designed to be immersed and to contain an angle gear box device comprising: two shafts rotating relatively to two perpendicular axes (XX′, YY′) whereof the first shaft (Y, Y′) is driven in rotation by said motor and a second shaft (XX′) drives at least a propeller in rotation; and mechanical elements comprising gears and roller bearings enabling the transmission of rotation of said first shaft to said second shaft. The invention is characterised in that it comprises two assembled matching half-shells (2) produced from a reinforced synthetic thermoplastic polymer material co-operating with each other along a joint plane defined by said axes (XX′ and YY′) of rotation of said rotary shafts and having each; a receased zone housing said angle gear box device in said joint plane, and a planar support surface (4a, 4b, 4c) external to said recessed zone and acting as support on the matching half-shell.