An abnormality detection method for an air jet loom, the air jet loom comprising: a plurality of main nozzles (142A, 142B); a plurality of sub-nozzles; a weft yarn passage into and through which a plurality of weft yarns (YA, YB) is inserted by air injections from the main nozzles (142A, 142B) and the sub nozzles; and a weft yarn arrival sensor disposed in the weft yarn passage, the abnormality detection method detecting traveling conditions of the inserted weft yarns (YA, YB) with the weft yarn arrival sensor, wherein the abnormality detection method comprises: inserting the plurality of the weft yarns (YA, YB), wherein, in each weft insertion, one of the plurality of the weft yarns (YA, YB) is drawn out by the air injection from a corresponding one of the main nozzles (142A, 142B) that is selected, and the weft yarn (YA, YB) is inserted into the weft yarn passage by the air injections from the sub-nozzles, the sub-nozzles and the weft yarn passage being commonly used for the plurality of the weft yarns (YA, YB); characterized in that the abnormality detection method further comprises detecting an abnormality that occurs in a sub-valve system when a bias angle of each of the plurality of the weft yarns (YA, YB) is out of a predetermined threshold range of the weft yarn (YA, YB), wherein the bias angle is a difference occurring between a weft yarn unwinding timing at which the weft yarn (YA, YB) is unwound from a weft yarn measuring and storing unit (130A, 130B) of the weft yarn (YA, YB) and a weft yarn arrival timing that is based on a weft yarn detection signal of the weft yarn arrival sensor. a plurality of main nozzles (142A, 142B); a plurality of sub-nozzles; a weft yarn passage into and through which a plurality of weft yarns (YA, YB) is inserted by air injections from the main nozzles (142A, 142B) and the sub nozzles; and a weft yarn arrival sensor disposed in the weft yarn passage, the abnormality detection method detecting traveling conditions of the inserted weft yarns (YA, YB) with the weft yarn arrival sensor, wherein the abnormality detection method comprises: inserting the plurality of the weft yarns (YA, YB), wherein, in each weft insertion, one of the plurality of the weft yarns (YA, YB) is drawn out by the air injection from a corresponding one of the main nozzles (142A, 142B) that is selected, and the weft yarn (YA, YB) is inserted into the weft yarn passage by the air injections from the sub-nozzles, the sub-nozzles and the weft yarn passage being commonly used for the plurality of the weft yarns (YA, YB); characterized in that the abnormality detection method further comprises detecting an abnormality that occurs in a sub-valve system when a bias angle of each of the plurality of the weft yarns (YA, YB) is out of a predetermined threshold range of the weft yarn (YA, YB), wherein the bias angle is a difference occurring between a weft yarn unwinding timing at which the weft yarn (YA, YB) is unwound from a weft yarn measuring and storing unit (130A, 130B) of the weft yarn (YA, YB) and a weft yarn arrival timing that is based on a weft yarn detection signal of the weft yarn arrival sensor. inserting the plurality of the weft yarns (YA, YB), wherein, in each weft insertion, one of the plurality of the weft yarns (YA, YB) is drawn out by the air injection from a corresponding one of the main nozzles (142A, 142B) that is selected, and the weft yarn (YA, YB) is inserted into the weft yarn passage by the air injections from the sub-nozzles, the sub-nozzles and the weft yarn passage being commonly used for the plurality of the weft yarns (YA, YB); characterized in that the abnormality detection method further comprises detecting an abnormality that occurs in a sub-valve system when a bias angle of each of the plurality of the weft yarns (YA, YB) is out of a predetermined threshold range of the weft yarn (YA, YB), wherein the bias angle is a difference occurring between a weft yarn unwinding timing at which the weft yarn (YA, YB) is unwound from a weft yarn measuring and storing unit (130A, 130B) of the weft yarn (YA, YB) and a weft yarn arrival timing that is based on a weft yarn detection signal of the weft yarn arrival sensor.