A breather device that is attached to a case of a transmission and performs ventilation in the case is provided. The breather device includes: a main body member including a storage portion with a closed bottom and an open upper end; and a cover member configured to close the upper end of the main body member and including a vent hole for ventilation to outside. The storage portion comprises a first chamber having, in a bottom portion, a communicating port communicating with an interior of the case, and a second chamber configured to receive a liquid entering from the vent hole. The communicating port allows oil to come and go between the case and the first chamber. The main body member is provided with a preventing part used for preventing the liquid from communicating between the first chamber and the second chamber.