Abstract: On the wall of a booster shell which defines a vacuum chamber, a nonreturn-valve casing is formed in two halves. One casing half is formed integral with the wall of the booster shell and the other casing half is rotatably fitted to the former. With the first casing half formed in effect as a part of the wall of the booster shell, there is no leakage of vacuum from the booster shell to the exterior thereof. The second casing half has an L-shaped connector pipe formed integral therewith for connection with a vacuum line and the bent tip portion of the L-shaped connector pipe can be set in any radial direction simply by adjusting the angular position of the second casing half relative to the first.