A transmission case housing a transmission mechanism for transmitting power to dirigible wheels includes an axle case having a main body and vertical bosses extending in a bent way from opposite ends of the main body, and pivoting cases supporting axles of the dirigible wheels. The axle case (8) is bisected into a first case half (8F) and a second case half (8R) at a split plane parallel to a direction of extension of the transmission mechanism (S) housed therein. The first case half (8F) and second case half (8R) are connected by connecting bolt (38). Each vertical boss (8b) has bearings (26), and each pivoting case (25) is attached to the bearings (26) to be pivotable relative to the vertical boss (8b).