An ink jet head comprising: a head substrate including thereon: a plurality of ink jet elements configured to cause ink to be ejected from a plurality of nozzles ; and a common wiring extending from an edge of the head substrate and electrically connected in common to the ink jet elements; a flexible substrate having a first edge coupled to the edge of the head substrate and including thereon: a drive circuit configured to drive the ink jet elements; a common connection wiring extending between the first edge and a second edge of the flexible substrate opposite the first edge and electrically connected to the common wiring ; a first reference potential wiring disposed separately from the common connection wiring , extending from the second edge of the flexible substrate , and electrically connected to a reference voltage terminal of the drive circuit; and an output monitor wiring extending from an output terminal of the drive circuit to the second edge of the flexible substrate through a region between the common connection wiring and the first reference potential wiring ; and a relay substrate coupled to the second edge of the flexible substrate and having a second reference potential wiring thereon, the second reference potential wiring being electrically connected to the common connection wiring and the first reference potential wiring , wherein the relay substrate further includes: a second output monitor wiring electrically connected to the output monitor wiring . a head substrate including thereon: a plurality of ink jet elements configured to cause ink to be ejected from a plurality of nozzles ; and a common wiring extending from an edge of the head substrate and electrically connected in common to the ink jet elements; a plurality of ink jet elements configured to cause ink to be ejected from a plurality of nozzles ; and a common wiring extending from an edge of the head substrate and electrically connected in common to the ink jet elements; a flexible substrate having a first edge coupled to the edge of the head substrate and including thereon: a drive circuit configured to drive the ink jet elements; a common connection wiring extending between the first edge and a second edge of the flexible substrate opposite the first edge and electrically connected to the common wiring ; a first reference potential wiring disposed separately from the common connection wiring , extending from the second edge of the flexible substrate , and electrically connected to a reference voltage terminal of the drive circuit; and an output monitor wiring extending from an output terminal of the drive circuit to the second edge of the flexible substrate through a region between the common connection wiring and the first reference potential wiring ; and a drive circuit configured to drive the ink jet elements; a common connection wiring extending between the first edge and a second edge of the flexible substrate opposite the first edge and electrically connected to the common wiring ; a first reference potential wiring disposed separately from the common connection wiring , extending from the second edge of the flexible substrate , and electrically connected to a reference voltage terminal of the drive circuit; and an output monitor wiring extending from an output terminal of the drive circuit to the second edge of the flexible substrate through a region between the common connection wiring and the first reference potential wiring ; and a relay substrate coupled to the second edge of the flexible substrate and having a second reference potential wiring thereon, the second reference potential wiring being electrically connected to the common connection wiring and the first reference potential wiring , wherein the relay substrate further includes: a second output monitor wiring electrically connected to the output monitor wiring . <EOS>