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, wherein the common connection wiring is formed along a third edge of the flexible substrate, the third edge being aligned perpendicular to the first and second edges; and 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; 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; and a dummy wiring on the flexible substrate, the dummy wiring disposed separately from the first reference potential wiring and extending between the first and second edges of the flexible substrate along a fourth edge of the flexible substrate opposite the third edge; characterised in that the dummy wiring is electrically isolated from any wiring on the head substrate and any wiring on the relay substrate. 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, wherein the common connection wiring is formed along a third edge of the flexible substrate, the third edge being aligned perpendicular to the first and second edges; and 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; 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, wherein the common connection wiring is formed along a third edge of the flexible substrate, the third edge being aligned perpendicular to the first and second edges; and 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; 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; and a dummy wiring on the flexible substrate, the dummy wiring disposed separately from the first reference potential wiring and extending between the first and second edges of the flexible substrate along a fourth edge of the flexible substrate opposite the third edge; characterised in that the dummy wiring is electrically isolated from any wiring on the head substrate and any wiring on the relay substrate.