The invention provides a ventilation structure for transmission which reliably discharges air after the air in a transmission case and oil are separated with a simple structure. A first chamber (11) for introducing thereinto the air inside a transmission case (1) from an introduction opening (H1), a second chamber (12) for leading out air from a lead-out hole (H2) to a vent valve (50), and a shielding part (13) for shielding the first chamber (11) and the second chamber (12) excluding a ventilation opening (S) so that the first chamber (11) is larger than the second chamber (12) in volume are formed adjacent to a plate member (31) in a first case (10). The introduction opening (H1) is formed by separating side surfaces of the first chamber (11) and the second chamber (12) using the plate member (31).