An air bag includes a bag portion formed by bonding two sheet members together. The bag portion includes an expansion portion including an annular joining portion extending in an annular shape, and a passage portion including a pair of parallel linear joining portions and connected to the expansion portion. Air filled in the bag portion flows through an internal space of the expansion portion surrounded by the annular joining portion and a passage sandwiched between the linear joining portions defining the passage portion. At a connection portion between the expansion portion and the passage portion, the linear joining portions extend in parallel, or the linear joining portions extend so as to expand toward a deep side of the passage from an opening position of the passage with respect to the internal space of the expansion portion.