In a breather device linking a space within a transmission case (1) to outside air, a breather (10) is provided at a middle position in a lengthwise direction on an upper portion of the transmission case, in particular, at a position displaced in an axial direction from a middle position in the lengthwise direction, and opens to outside air via a ventilating mechanism (2) that extends in a direction opposite this displaced direction. Thus, through the combination of displacement in a lengthwise direction of the position of the breather with respect to a middle portion in a lengthwise direction of the transmission case and the direction of the ventilating mechanism facing in a direction opposite that direction, the occurrence of breather blow is prevented during both forward and rearward axis tilting of the transmission by an action where a forward side of the ventilating mechanism lowers when,the position of the breather tilts in a direction away from the oil level, and conversely, where the forward side of the ventilating mechanism rises when the position of the breather tilts in a direction towards the oil level.