Abstract: Provided is a thickness variation adjustment-type air ring capable of accurately and optimally controlling the volume of cooling air at all positions in the circumferential direction, and stably manufacturing a synthetic resin film having an exceptionally low thickness variation. A thickness variation adjustment-type air ring is provided on a die of an inflation film manufacturing apparatus which locally controls the volume of cooling air discharged from a cooling air passage in a circumferential direction to adjust the thickness variation of a synthetic resin film when blowing the cooling air around a molten synthetic resin tube extruded in a longitudinal direction to cool and solidify the molten synthetic resin tube to form the synthetic resin film, the thickness variation adjustment-type air ring.