An axle driving unit includes an axle, a hydraulic pump, a hydraulic motor having a motor shaft on which an output gear is provided to be drivingly connected to said axle, a center section having a pump mounting surface and a motor mounting surface onto which the hydraulic pump and the hydraulic motor are mounted respectively so as to be fluidly connected with each other through the center section, a housing constituted by a first housing part and a second housing part joined to each other at a joint surface, a projection integrally formed on the center section so as to be fitted to the housing, thereby fixing a position of the center section relative to the housing, and a fastener fastening the center section to the first housing part. The pump mounting surface and said motor mounting surface may be disposed perpendicular to each other. Said pump mounting surface and said motor mounting surface are disposed perpendicular to each other. The projection may be extended in parallel to the pump mounting surface and perpendicular to the motor mounting surface.