PURPOSE: To lower the center of gravity of a mission case with preventing damage by entering the transmission case into a depression part of upwardly concave having a bottom installed in front part of center float and opening mud-ejecting hole at rear side position of the depression part.

CONSTITUTION: Transmission case 1 is entered into a depression part 15a of upwardly concave having a bottom installed in a front part of a center float 15 to lower center of gravity of the transmission case and prevent damage of bottom face of the transmission case. A mud-ejecting hole 15d is opened at a lower face side of the rear side position of the depression part 15a to prevent stagnation of mud in the depression part.