An inflator includes: an inflator case configured from a bottomed cylindrical upper case and lower case, with the inflator case having an upper combustion chamber and a lower combustion chamber in which gas generants are accommodated, the upper combustion chamber and the lower combustion chamber being partitioned by a partition wall, and with the inflator case having a peripheral wall in which upper gas discharge holes and lower gas discharge holes are formed; a first inner cylinder member in which a transfer charge for combusting the gas generant inside the lower combustion chamber is accommodated; a second inner cylinder member whose upper end portion penetrates the partition wall and is positioned in a lower portion of the upper combustion chamber, with a transfer charge for combusting the gas generant inside the upper combustion chamber being accommodated inside the second inner cylinder member; a first igniter and a second igniter.