A method of determining amplitude of reed vibration for an air jet loom, wherein the air jet loom determines the amplitude of the reed vibration that is vibration of a reed based on detected signals detected by a sensor for determining the amplitude of the reed vibration, is characterized by disposing the sensor within a weaving width in a direction of weft insertion, wherein the sensor is a reflection optical sensor that is insertable into a shed between warp yarns and retractable from the shed and determining the amplitude of the reed vibration within the weaving width based on amplitude ofa signal waveform S1 contained in the detected signal V2 detected during a period when no weft yarn is present in a weft passage among the detected signals V1-V4 detected by the sensor.