A vacuum booster is configured so that at least when an input rod is released, a front surface of a flexible portion of a valve element communicates with a first port which communicates with a vacuum chamber. A valve element holder is formed of a single member made of a sheet of steel which includes a rim portion, an annular arm portion, and a crimp holding portion. The rim portion is fitted to an inner circumferential surface of a valve cylinder via a seal member. The annular arm portion extends inwards in a radial direction from a front end of the rim portion. The crimp holding portion extends from an inner circumferential end of the arm portion to form a bag-like shape and to narrow a front opening portion thereof. The crimp holding portion crimp holds a mounting bead portion of the valve element from rear thereof.