Abstract: A tandem type brake booster is disclosed which comprises a booster body, a partitioning wall partitioning the interior of the body into front and rear chambers, a valve body axially movably disposed in the body, front and rear power pistons provided with respective diaphragms associated therwith and respectively dividing the interior of the front and rear chambers into respective constant and variable pressure chambers, a plunger axially movably disposed in the valve body, and a valve mechanism disposed in the valve body to selectively introduce atmospheric air and negative pressure into the variable pressure chambers. The valve body includes a large diameter portion on which the front and rear power pistons are secured and a small diameter portion, the large diameter portion being provided adjacent to the rear end thereof with a slightly reduced diameter stepped portion, the small diameter portion being provided with a circumferentially extending groove.