To provide a fuel vapor leakage detecting device capable of securing airtightness of a switching valve.

A housing 40 is provided with an open air port 41 for communicating a housing inside space 401 with the atmosphere. Assuming a point on an opening surface 227 of a pump discharge port 221 for discharging air by a pump 22 as a first point P1, a point on an opening surface 411 of the open air port 41 as a second point P2 and a straight line of connecting the first point P1 and the second point P2 as a first virtual straight line L1, the open air port 41 is formed in a position of not crossing between the first virtual straight line L1 and the switching valve 30. When performing fuel vapor leakage detecting treatment, dust generated by the pump 22 is discharged to the housing inside space 401 from the pump discharge port 221, and is discharged to the atmosphere via the open air port 41 without passing through the vicinity of the switching valve 30. Thus, the dust does not stick to a first valve seat 351 and a washer 352 of the switching valve 30. Thus, the airtightness of the switching valve 30 can be secured.