Abstract: An improvement of a brake booster is provided which is also provided with the function to serve as an automatic braking unit. A front bead of bellows disposed in a constant pressure chamber is supported by and urged against the wall surface of a front shell by a support of a support member. When the atmosphere is introduced into the internal space within the bellows to cause the brake booster to serve as an automatic braking unit, even though the front bead of the bellows is axially pulled as the bellows expands, a leakage around a seal where the front bead is connected is prevented because the front bead is urged against the wall surface of the front shell by the support member.