A vacuum type brake booster device is provided which comprises a booster shell of substantially elliptical shape in transverse cross section, a booster piston of similar shape as the booster shell and accommodated therein for axial reciprocating movement, and a diaphragm secured about the outer periphery thereof to the booster shell and about the inner periphery to the booster piston to divide the interior of the booster shell into a first working chamber in communication with a vacuum source and a second working chamber adapted to be selectively placed in communication with the first working chamber or the atmosphere by a switching valve. The diaphragm is also shaped substantially elliptically in transverse cross section, and includes a pair of opposite straight peripheral wall sections and a pair of opposite arcuate peripheral wall sections interconnecting the straight wall sections; the arcuate wall sections have a wall thickness greater than that of the straight wall sections.