To firmly fit a negative pressure actuator 2 to a duct 1, without using auxiliary small screws or the like.

The duct 1 made of a synthetic resin has a flat fitting seat 31, on which nearly rectangular opening 32 is formed. The negative pressure actuator 2 has a synthetic resin-made case 11, on the lower face of which a neck part 42 being projectingly formed and a locking piece 43 also is formed. A locking groove for tightly holding the peripheral edge of the opening 32 is formed between the locking piece 43 and the lower face of the case 11. The actuator 2 is fixed, by inserting the neck part 42 thereof into the opening 32 and then making it rotate by 90 degrees. Furthermore, a positioning recess 34 and a positioning projection 36 are formed at the duct 1 side of the fitting seat 31, and a circular arc-shaped projection 53 and a positioning hole 52 are formed at the actuator 2 side, respectively as the positioning parts for rotational direction. As a result of their engagement, the actuator 2 is prevented from loosening.