Gas generator

A gas generator includes a cup where a gas generating agent is accommodated and a holder. The holder includes a first recessed portion and an annular groove provided in an end surface on a side facing the cup, a second recessed portion provided in an end surface on an opposite side, and a partition between the first and second recessed portions. A bottom surface of the first recessed portion includes a first and second bottom surface located on a side of the second recessed portion. The gas generator satisfies A