Abstract: In cases where a defective weaving caused by yarns occurs during operations of a loom, a real cause of defect is identified as to whether it arises from yarns or from setting of the devices on the loom. A display unit of a loom for switching operation modes of a plurality of devices on the loom in response to the rotation of a main shaft of the loom comprises a sensor for detecting a defective weaving, a counter for counting weft picking number with the rotation of the main shaft of a loom, a operation signal generation means for outputting operation signals to the plurality of devices on the loom, in response to the weft picking number and set operation modes, and a display means for displaying information concerning content of defective weaving which is detected by a sensor and information concerning operation signals when the defective weaving occurs.